Microwave-Assisted Solvothermal Synthesis of Fe₃O₄/CeO₂ Nanocomposites and Their Catalytic Activity in the Imine Formation from Benzyl Alcohol and Aniline

Abstract

Fe₃O₄/CeO₂ nanocomposites were synthetized by coating magnetite seeds of different morphologies (hexagonal, spheroidal, quasi-spherical) with ceria, in ethylene glycol as solvothermal solvent. The synthesis was performed in the presence of microwave irradiation aiming to overcome the common disadvantages proper of the classic solvothermal/hydrothermal procedure. The obtained nanocomposites were calcined at the optimum temperature of 550 °C. The structure of the new nanomaterials was carefully investigated by IR, XRD, SEM, EDS and TEM analyses. The nanocomposites resulted to be constituted by CeO₂ nanoparticles distributed onto Fe₃O₄ seeds, that kept their pristine morphology. The new materials were used as catalysts for imine synthesis from benzyl alcohol and aniline. The highest imine conversion rate was obtained with Fe₃O₄/CeO₂, which was synthesized from Fe₃O₄ nanoparticles (hexagonal) obtained by microwave hydrothermal procedure in the absence of any organic additive (polyvinylpyrrolidone, trisodium citrate dihydrate or oleic acid). The catalyst could be easily removed from the reaction mixture with the help of an external magnet, and it was recycled for at least five runs with increasing catalytic activity.

1. Introduction

Ceria is an oxide of the most abundant rare-earth metal of the lanthanide series, having the unique feature to pass from +4 to +3 oxidation state based on the occurrences [1]; thus, ceria exists as CeO₂ containing also Ce₂O₃. CeO₂ nanoparticles have highly reactive surface area due to presence of oxygen vacancies in their structures, acting as free radical scavengers [2], and they are useful for several applications, such as, biomedical [3,4], biological [5], gas sensing [6], catalysis [7], energy production [8] and optical devices [9]. Nanoceria can be synthetized following several methods [3,4,5,10,11], i.e., by precipitation [12], green synthesis [13], hydrothermal [14,15], solvothermal [16], microwave-assisted [17,18] and sonochemical [19] procedure, and the way of synthesis strongly influences the morphological properties, thus the activity and reactivity of the obtained nanomaterial [20,21]. In the field of catalysis, nanoceria has been used as efficient catalyst for aerobic oxidation reactions due to its oxygen storage capability [22], although its recovery at the end of the catalytic cycle cannot be carried out by simple filtration, because of its too tiny consistency [23]. A method to overcome this disadvantage is to anchor nanoceria to magnetite (Fe₃O₄) [24,25,26,27,28,29,30], rendering the whole material easy to be removed from the reaction mixture by using an external magnet. So far few types of magnetic Fe₃O₄/CeO₂ nanocomposites with core-shell structure have been synthetized [31,32,33,34] and some of them have the problem of iron leaching during catalysis, which can be overcome by the addition of an organic polymer layer (polysulfone) [32] or a silica interlayer. In details, Chen and co-workers [31] synthetized Fe₃O₄/SiO₂/CeO₂ with the precipitation method, followed by calcination at 500 °C, obtaining nanoparticles with mean diameter of 150 nm; Wei et al. [32] prepared Fe₃O₄/CeO₂ of ca. 200 nm in size with the solvothermal/hydrothermal method; Rossi and co-workers [34] were not able to coat nanoceria directly onto magnetite nanoparticles without using a SiO₂ interlayer, and, finally, very small core-shell magnetite/ceria nanoparticles (50 or 20 nm size diameter, depending on the precipitating agent used) were synthetized by Wu et al. [33] with the reverse micelle systems and by Gawande and co-workers [35] with the wet impregnation protocol. Recently, in our research group, a simple and fast microwave-assisted hydrothermal procedure has been developed for the synthesis of magnetite nanoparticles, controlling both their morphology and growth [36]. Specifically, quasi-spherical (25 nm in size) and spheroidal (60 nm in size) Fe₃O₄ nanoparticles have been obtained by adding oleic acid and trisodium citrate to the reaction mixture, respectively; regular hexagonal magnetite nanoplatelets with a facet to-facet distance of 120 nm and a thickness of 53 nm and 40 nm have been recovered by using polyvinylpyrrolidone, as additive, or no additive, respectively. Following our studies on magnetite nanoparticles [36], we decided to add nanoceria to these Fe₃O₄ nanoparticles, to obtain Fe₃O₄/CeO₂ nanocomposites with different morphologies of magnetite seeds. The preparation method proposed in the present work is the microwave-assisted solvothermal synthesis in the presence of ethylene glycol. To the best of our knowledge this procedure has never been used before for the formation of Fe₃O₄/CeO₂ nanocomposites.

The aim of this study was to verify whether: (i) the nanoceria deposition process changes the pristine morphology of the magnetite nanoparticles, (ii) the different morphologies of magnetite seeds and the use of organic stabilizer (polyvinylpyrrolidone, trisodium citrate dihydrate or oleic acid) during preparation of the catalyst can influence its catalytic activity. The imine formation from benzyl alcohol and aniline was taken as the model reaction for catalytic tests.

Recently it has been reported that CeO₂ is the most active heterogeneous catalyst among various metal oxides tested (such as Al₂O₃, TiO₂, ZnO, etc.) for imine synthesis from alcohol and amine under air [37]. Furthermore, it has been proven that the efficiency of ceria for the synthesis of imine (whose importance stands in its role as intermediate for the preparation of various biological, agricultural and pharmaceutical compounds) is directly correlated with the number of oxygen vacancies onto the catalyst surface [38]. In particular, nanoceria prepared with the solvothermal method using ethylene glycol as the solvent has been found to have a number of oxygen vacancies higher than nanoceria synthetized with the hydrothermal procedure [38]. However, the poor recyclability of nanoceria, due to the difficulty of recovering caused by the highly powdery consistency of the nanomaterial, has limited its use as an efficient catalyst for imine formation. The use of the here presented Fe₃O₄/CeO₂ nanocomposites should overcome this problem, because the nano-catalysts are easily removable from the reaction mixture using an external magnet. In addition, the ceria nanoparticles deposited onto the magnetite seeds were prepared in our laboratory by avoiding the use of water as the solvent, i.e., under the best conditions for increasing the number of oxygen vacancies onto the ceria surface. All these features render the proposed synthesis procedure highly promising in the field of catalysis.

2. Results and Discussion

2.1. Synthesis of Magnetite/Ceria Nanocomposites

The deposition of ceria nanoparticles onto magnetite seeds was carried out by solvothermal method assisted by microwave irradiations, in order to overcome the common disadvantages proper of the classic solvothermal/hydrothermal procedure [39], i.e., difficult temperature control, slowness of the process, production of relatively large nanoparticles (mean diameter in the range 200–800 nm). The use of microwaves in the preparation of nanostructured materials has the advantage to increase the crystallinity of the nano powders and decrease the process times, and to favor homogeneous nucleation mechanisms leading to a narrow distribution of nanoparticle dimensions [40]. The magnetite seeds synthesis by microwave-assisted hydrothermal procedure led to morphologically different Fe₃O₄ nanoparticles, depending on the stabilizer used during their synthesis (polyvinylpyrrolidone (PVP), trisodium citrate dihydrate (CITR) and oleic acid (OA)) [36]. Their main features are reported in

Table 1. Morphological features of Fe₃O₄ nanoparticles synthetized by microwave-assisted hydrothermal procedure [36] ^a Corresponding to the facet to facet distance.

Entry	Sample	Stabilizer	Shape	Mean Thickness a (nm)	Mean Diameter (nm)
1	Fe3O4	–	Hexagonal	40	120 a
2	Fe3O4:PVP	PVP	Hexagonal	53	116 a
3	Fe3O4:CITR	Trisodium citrate	Spheroidal	–	60
4	Fe3O4:OA	Oleic acid	uasi-Spheres	–	25

.

We decided to deposit nanoceria onto the nanoparticles reported in Table 1 by using the microwave-assisted solvothermal procedure (ethylene glycol as the solvent) to accelerate growth kinetics. All samples reported in Table 1 were subjected to ceria deposition, by using two different Ce/Fe₃O₄ molar ratios (0.6 and 1.5), followed by calcination under nitrogen (Scheme 1).

2.2. Thermogravimetric and Differential Scanning Calorimetry (TGA/DSC) Analyses

Figure 1 shows graphics obtained by thermogravimetric and differential scanning calorimetry (TGA/DSC) analyses of the nanocomposites (before calcination) obtained starting from the magnetite samples reported in Table 1, using a Ce/Fe₃O₄ molar ratio equal to 0.6 and 1.5, respectively.

TGA/DSC analyses revealed that, apart for the solvent mass loss, all samples underwent large weight loss in the temperature range 280–850 °C (13–26%) accompanied by endothermic processes [41], probably due to oxalate decomposition and leaching of organic ligands [38], that formed an inorganic–organic composite with ceria. Oxalate formation can be ascribable to the partial conversion of ethylene glycol to oxalic acid at high temperatures. Under the same reaction conditions, the cerium salt decomposed forming CeO₂, whose superficial hydroxy groups reacted with oxalic acid, giving rise to the above-mentioned polymeric inorganic-organic compound. Similar TGA behavior was observed by Yang and co-workers in nanoceria prepared by solvothermal synthesis in ethylene glycol [38]. However, DSC curves (Figure 1) show that all samples lose most of their weight around 500 °C due to CO and CO₂ emissions. Moreover, by increasing the temperature up to 1000 °C, a sintering peak in the DSC curve sometimes appears (see for example Figure 1g), indicative for sample melting, that compromises the nature and therefore the functionality of the studied material. This phenomenon strongly suggests choosing a calcination temperature not higher than 550 °C. Finally, TGA/DSC curves of 0.6CeO₂ and 1.5CeO₂ (Figure S1) showed the same trend observed for all magnetite-ceria nanocomposites in the range 280–500 °C, due to oxalate decomposition and leaching of organic ligands. At temperatures higher than 550 °C the sintering processes are not observed due to the absence of magnetite.

2.3. IR Analyses

The observations revealed by TGA/DSC analyses were confirmed by IR studies. In fact, coordination of ethylene glycol and its decomposition products can be further proven by IR spectroscopy. Figure 2 shows IR spectra registered on all samples as prepared and recovered after TGA analysis, i.e., after calcination at 1000 °C under N₂ flow. IR spectra of all samples before calcination display a broad peak belonging to O–H stretching around 3417–3450 cm⁻¹, as well as a strong signal due to the asymmetric stretching of the O–C=O oxalate moiety in the range from 1632 cm⁻¹ to 1602 cm⁻¹, and a sharp peak at ca. 1080 cm⁻¹, which corresponds to C–O vibrations, proving the coordination of the ethylene glycol to Ce through Ce–O–C bonds. Only spectra reported in Figure 2g,h before calcination show two characteristic peaks at 2918 and 2850 cm⁻¹, due to oleic acid C-H stretching. In addition, all IR spectra before calcination show both the Ce–O (470–490 cm⁻¹) stretching signals [42] and typical magnetite peaks, falling around 360, 450 and 580 cm⁻¹ [43]. After calcination at 1000 °C, in all IR spectra, peaks due to organic moieties disappear and signals due to Fe₃O₄ structures modify, being shifted or vanishing, confirming what already found in TGA/DSC analyses, i.e., loss of the carbon ligands (at T > 550 °C) and modification of the Fe₃O₄/CeO₂ nanocomposite structure (at temperatures higher than 850 °C).

However, even after calcination at 1000 °C, all samples resulted magnetically active.

IR spectra of 0.6CeO₂ and 1.5CeO₂ (i.e., CeO₂ nanoparticles prepared under the same procedure of Fe₃O₄/0.6CeO₂ and Fe₃O₄/1.5CeO₂ respectively, but in the absence of magnetite), before and after calcination at 1000 °C, are reported in Figure S2 for comparison purposes. Before calcination, all peaks belonging to the formation of an organic-inorganic layer are evident. After calcination, the presence of only Ce–O stretching signals could be observed.

2.4. XRD Analyses

The as prepared and calcined (at 1000 °C) samples were also characterized by XRD. Aiming at recognizing the diffraction peaks belonging to neat CeO₂ prepared with the solvothermal method, the not-containing Fe₃O₄ seeds, i.e., 1.5CeO₂ and 0.6CeO₂ batches, were subjected to XRD analyses prior to all other nanocomposites. Figure 3 reports the diffraction peaks of 0.6CeO₂ and 1.5CeO₂ samples, before and after calcination at 1000 °C under nitrogen flow.

Figure 3 shows peaks at 28.0°, 32.7° and 47.0° 2θ corresponding to the (111), (200) and (220) crystal planes of ceria, respectively. They are sharp only in the XRD spectra of the calcined samples (Figure 3c,d), because the CeO₂ structure transition from the amorphous to the crystalline phase occurs at temperatures higher than 500 °C [42]. In addition, weak signals at 12.9 and 17.9° 2θ due to cerium (III) oxalate hydrate are present only before calcination (Figure 3a,b), suggesting that the organic cerium ligands, coming from the preparation step in ethylene glycol, decomposed at high temperatures.

Figure 4 reports the diffraction peaks of Fe₃O₄/0.6CeO₂ and Fe₃O₄/1.5CeO₂, before and after calcination at 1000 °C under nitrogen flow.

In the XRD spectra of Fe₃O₄/0.6CeO₂ (Figure 4a) and Fe₃O₄/1.5CeO₂ (Figure 4b), only the diffraction peaks belonging to magnetite are recognizable [36], and no ceria sharp signals are visible, as expected on the base of XRD spectra of 0.6CeO₂ (Figure 3a) and 1.5CeO₂ (Figure 3b) before calcination. After calcination at 1000 °C (Figure 4c,d), surprisingly no diffraction peak of crystalline ceria can be found. Instead of them, the spectra report the characteristics peaks of cerium ferrite, which formed during sintering processes occurring at temperatures higher than 600 °C. Moreover, in both calcined samples, wüstite (instead of magnetite) is detected with its characteristic peaks at 36.1°, 42.0° and 60.9° 2θ, belonging respectively to the (111), (200) and (220) planes (Figure 4c,d). Weak peaks of magnetite are detected only in the calcined Fe₃O₄/0.6CeO₂ composite (Figure 4c).

Figure 5 and Figure 6 show XRD spectra of all eight nanocomposites calcined at 1000 °C. Notably, signals belonging to wüstite and cerium ferrite can be observed in all of them.

2.5. TEM Analyses

TEM images of Fe₃O₄/0.6CeO₂ (Figure 7a) and Fe₃O₄/1.5CeO₂ (Figure 7b) revealed that the original hexagonal morphology of the magnetite seeds was retained. However, a close inspection of the TEM image of Fe₃O₄/0.6CeO₂ (Figure 7a) highlighted that hexagonal magnetite seeds were not sticked to nanoceria. On the contrary, in the case of Fe₃O₄/1.5CeO₂ (Figure 7b) nanoceria was adherent to Fe₃O₄ nanoparticles. This evidence suggested us to consider, for possible catalytic purposes only, nanocomposites obtained by using a Ce/Fe₃O₄ molar ratio equal to 1.5.

TEM images of calcined (at 1000 °C) Fe₃O₄/0.6CeO₂ (Figure S3a) and Fe₃O₄/1.5CeO₂ (Figure S3b) confirmed what found with other analyses, i.e., the sintering of cerium and iron compounds at nanoscale levels. The occurrence of melting processes at high temperatures were also revealed by FESEM image of Fe₃O₄/1.5CeO₂ calcined at 1000 °C (Figure 7c), where the hexagonal nanoparticles of magnetite are not visible anymore.

2.6. Calcination at 550 °C

All the discussed analyses revealed the occurrence of sintering processes in all nanocomposites calcined at 1000 °C. Moreover, TGA/DSC experiments showed that this degradation starts at temperature higher than 600 °C. In addition, TEM images evidenced the poorness of the amount of ceria in nanocomposites prepared by using a Ce/Fe₃O₄ molar ratio equal to 0.6. For these reasons, only samples synthetized with a Ce/Fe₃O₄ molar ratio equal to 1.5 were considered for further studies (including catalytic tests), and a new calcination temperature was chosen. It was set at 550 °C, as the best value, because it is lower than 600 °C in order to avoid sintering progressions, but high enough to allow the transition of ceria from the amorphous to the crystalline phase [44].

Thus, all nanocomposites synthetized with 1.5 Ce/Fe₃O₄ molar ratio were calcined at 550 °C and subjected to XRD, IR, FESEM and EDS analyses.

Their XRD spectra (Figure 8) show diffraction peaks belonging to both ceria and magnetite, besides the absence of signals due to sintering compound (cerium ferrite). Notably, the particle sizes of crystalline CeO₂ calculated by the Scherrer formula (Figure 8) range between 4.1 and 4.9 nm, highlighting their nanoscopic dimension. On the contrary, the particle size of the same samples calcined at 1000 °C ranges from 37.7 to 56.7 nm, due to the occurrence of sintering processes.

A perusal of XRD patterns led us to the following observations on the nanocomposites prepared with the addition of OA (green lines of Figure 5, Figure 6 and Figure 8): (i) XRD patterns of OA samples after calcination at 1000 °C (Figure 5 and Figure 6) show the presence of crystalline CeO₂, whereas other samples do not; (ii) OA samples show the presence of Fe-metal (particularly in Figure 5); (iii) after calcination at 550 °C, the OA sample shows much less magnetite (Figure 8). All these occurrences, together with the observation that the weight loss, as determined by TGA (Figure 1) being much higher for both OA samples (Figure 1g,h), are indicative for the presence of much more organic compounds with respect to the other prepared nanocomposites. From these observations, it can be concluded that a higher amount of pyrolysis products from oleic acid and/or ethylene glycol is present in OA samples. This seems to have a significant influence on phase formation. An effect on the ‘melting’ in TGA curve of sample g) (Figure 1) could also be possible and partial carbothermal reduction of iron oxide to metal iron can be promoted. In fact, thermodynamically, the reduction of iron oxide occurs in steps. Accordingly, the following stable iron oxides can form during the reduction (above 570 °C): hematite, magnetite and non-stoichiometric wüstite. During the stepwise reduction of magnetite to wüstite, metallic iron is generally produced [45]. TGA/DSC analyses of magnetite/ceria nanocomposites (Figure 1) revealed the occurrence of a carbothermal reduction of magnetite: wüstite was first formed followed by metallic iron. Direct reduction of iron oxides by solid carbon has also been reported and the carbon source could be coke, graphite, coal, char, deposited carbon and activated carbon [46]. The presence of carbon can change the activation and the rate of the reduction of magnetite to wüstite and metallic iron. In fact, by TGA/DSC analyses of Fe₃O₄:OA/0.6CeO₂ and Fe₃O₄:OA/1.5CeO₂ (Figure 1g,h), a high content of organic ligands were detected due to higher weight loss in the temperature range between 350 °C and 450 °C with respect to the other magnetite/ceria nanocomposites. The presence of a high content of carbon in the Fe₃O₄:OA/0.6CeO₂ and Fe₃O₄:OA/1.5CeO₂ samples led to a more pronounced endothermic peak at ca. 750 °C ascribable to the carbothermal reduction [45].

All nanocomposites calcined at 550 °C were analyzed by IR spectroscopy. IR spectra (Figure 9) display typical peaks of ceria and magnetite in the range of 360–580 cm⁻¹ and the absence of signal ascribable to any organic ligands (over the detection limit of the analytical technique), proving the loss of the majority of the organic layer and the presence of both CeO₂ and Fe₃O₄ in the nanocomposites.

Finally, all samples calcined at 550 °C were investigated by FESEM spectroscopy. FESEM micrographs (Figure 10) show that the pristine morphology of magnetite nanoparticles was kept after ceria deposition. In fact, in both Fe₃O₄/1.5CeO₂ and Fe₃O₄:PVP/1.5CeO₂ (Figure 10a,c) the hexagonal structure observed in Fe₃O₄ and Fe₃O₄:PVP seeds was maintained [36], while Fe₃O₄:CITR/1.5CeO₂ and Fe₃O₄:OA/1.5CeO₂ (Figure 10b,d) showed an almost or completely spherical structure, as well as their respective precursors [36].

In addition, the presence of ceria nanoparticles was confirmed by EDS analyses (Figures S4–S7), but the core-shell structure was not observed. All materials were magnetite/ceria nanocomposites, in which the pristine morphologies of Fe₃O₄ seeds were retained after CeO₂ deposition. EDS elemental analyses revealed the following CeO₂/Fe₃O₄ mass ratio in the nanomaterials calcined at 550 °C: 14/86, 18/82, 21/79, 13/87 in Fe₃O₄/1.5CeO₂, Fe₃O₄:CITR/1.5CeO₂ Fe₃O₄:PVP/1.5CeO₂ and Fe₃O₄:OA/1.5CeO₂, respectively. Furthermore, EDS spectra (Figures S4–S7) also showed the presence of C and O, as well as the absence of N in all samples. The presence of C residual species would affect the catalytic activity of the nanocomposites (see the next paragraph).

The calcination temperature also affected the porosity of the nanocomposites, as highlighted by Figure 11, reporting the change of morphology of Fe₃O₄/1.5CeO₂ by passing from the pristine state to the forms after calcination at 550 °C and 1000 °C, respectively. The mesoporous structure observed after calcination at 550 °C should be useful for catalytic purposes, as it facilitates the arrival of substrates and/or intermediates to the catalytically active sites.

2.7. Catalytic Tests

All nanocomposites synthetized with a Ce/Fe₃O₄ molar ratio equal to 1.5 and calcined at 550 °C were tested as catalysts in the aerobic imine formation reaction, starting from aniline and benzyl alcohol under solvent less conditions (Scheme 2). The catalytic test was designed on the base of Tamura [37] experiments by using benzyl alcohol (0.2 mmol) and aniline (0.4 mmol) in the presence of 10 mg CeO₂ without solvent at 323 K. As suggested by Yang [38], the reaction rates were employed for assessing the catalytic ability, calculating the imine formation amount per hour per ceria amount (mmol h⁻¹g⁻¹).

Figure 12 shows the catalytic activity of the various nanocomposites employed for the imine formation reaction under the conditions reported in Scheme 1. Negligible conversion of the substrate was observed in the absence of any catalyst, while a good rate of 1.34 mmol h⁻¹g⁻¹, comparable to the value reported by Yang [38], was obtained in the presence of Fe₃O₄/1.5CeO_2. Unsatisfactory imine formation rate values of 0.16 mmol h⁻¹g⁻¹, 0.58 mmol h⁻¹g⁻¹ and 0.63 mmol h⁻¹g⁻¹ were observed for Fe₃O₄:CITR/1.5CeO₂, Fe₃O₄:PVP/1.5CeO₂ and Fe₃O₄:OA/1.5CeO₂, respectively. This means that the reaction rate for imine synthesis from benzyl alcohol and aniline over various nanocomposites was the best in the case of the catalyst obtained without any organic stabilizers. It can suggest that the part of catalytically active centers might be blocked by coke deposited on the surface of catalysts.

The recyclability tests were carried out only with the most active catalyst, i.e., Fe₃O₄/1.5CeO_2. Thus, at the end of the reaction, Fe₃O₄/1.5CeO₂ was removed from the reaction mixture by the use of an external magnet, then it was washed with ethanol and calcined under air at 400 °C for 3 h, before being used for a subsequent run. This calcination process was necessary to clear the oxygen vacancies (i.e., the catalytically active sites) possibly occupied by water oxygen [47], since water was contained in the ethanol solvent used during the recovery work-up of the catalyst. In fact, in the absence of the calcination step before reuse, the catalytic activity of the nanocomposite dramatically dropped to 0.39 mmol h⁻¹g⁻¹ for the presence of hydroxylic groups on the ceria surfaces. Figure 13 reports the recyclability of Fe₃O₄/1.5CeO₂ calcined under air at 400 °C for 3 h before each subsequent catalytic cycle. The activity of the catalyst increased with the reuses reaching imine formation rate of 2.3 mmol h⁻¹g⁻¹ and the catalyst resulted magnetically active after each run. For comparison purposes, the catalytic activity of commercial nanoceria, calcined at 400 °C for 3 h under air before use, was studied. The observed reaction rate was 0.34 mmol h⁻¹g⁻¹ (Figure 13), even lower than the value reached with Fe₃O₄/1.5CeO₂ in the first run, proving that the catalyst morphology strongly affects its catalytic activity.

In addition, Fe₃O₄/1.5CeO₂ nanocomposite was more active than other commercial metal oxides for catalyzing the considered model reaction (Table S2).

To get insight into the structure changes of the nanocomposite with the recycles, Fe₃O₄/1.5CeO₂ recovered after five subsequent runs was subjected to XRD analyses. The diffraction pattern of the used catalyst (Figure 14) resembles the one reported in the XRD spectrum of the same nanocomposite before its use in catalysis (Figure 8a), showing the typical peaks belonging to ceria and magnetite. Both XRD spectra (Figure 8a and Figure 14) display a very broad signal below 17.9°, which can contain diffraction peaks of magnetite, cerium (III) oxalate hydrate and even iron (II) oxalate dihydrate (humboltite). In the XRD spectrum of the recovered catalyst (Figure 14), ceria peaks are more intense than the broad signal below 17.9°, while in the spectrum of the pristine catalyst their intensities are similar, thus explaining the increasing of the catalytic activity with the reuses, probably due to degradation of the organic impurities (oxalates) coming from the solvothermal synthesis. In fact, all organic ligands (oxalate, ethylene glycol derivatives, etc.) formed during the preparation of the catalyst, can occlude the catalytically active sites. Their removal might benefit the activity of the whole catalyst.

The plausible mechanism [37] commonly accepted for the imine synthesis over ceria is reported in Scheme 3 and it is based on the redox properties of ceria, because the oxidative dehydrogenation of benzyl alcohol occurs when cerium is reduced from +4 to +3 oxidation state. Afterwards, benzaldehyde reacts with aniline to afford imine, and cerium is in turn oxidized from +3 to +4 oxidation state by dioxygen to start a new cycle. In addition, Zhang et al. [38] have already demonstrated that the catalytic activity of CeO₂ in the oxidative imine formation reaction is related to oxygen vacancies and Ce³⁺ amount in the surface of the catalyst.

FT-IR spectrum of Fe₃O₄/1.5CeO₂ after calcination for 3 h at 400 °C (Figure S8) displays typical peaks of ceria and magnetite in the range of 360–580 cm^–1. O–H stretching broad signal are still visible because calcination and subsequent cooling were carried out under air. Although it has been demonstrated that calcination under inert atmosphere leads to a higher number of oxygen vacancies over ceria surface than calcination under air [2], in the case of Fe₃O₄/1.5CeO₂, air is necessary during calcination, because it burns coke residues that may occlude catalytically active sites.

3. Experimental

3.1. Materials and Methods

Tap water was de-ionized by ionic exchange resins (Millipore, Darmstadt, Germany) before use. Commercial ferrous chloride tetrahydrate (99.9%,VWR, Milan, Italy), ceric ammonium nitrate (98.5%, VWR, Milan, Italy), ceria nanoparticles CeO₂ (<25 nm, Sigma-Aldrich, Milan, Italy), sodium hydroxide pearls (99.0%, Carlo Erba, Milan, Italy), hydrazine monohydrate (98.0%, Sigma-Aldrich, Milan, Italy) 64–65% (v/v) aqueous solution, oleic acid (OA, 90%, Alfa-Aesar, Kandel, Germany), trisodium citrate dihydrate (CITR, 99%, Sigma-Aldrich, Milan, Italy), polyvinylpyrrolidone (PVP, MW = 40,000 Da, Sigma-Aldrich, Milan, Italy) and absolute ethanol (99.5%, Sigma-Aldrich, Milan, Italy) were used as received.

Microwave-assisted solvothermal syntheses were carried out in an ETHOS E-TOUCH Milestone (Sorisole (BG), Italy) applicator, using the conditions described in the following Section 3.2.

Catalytic reactions were monitored by GLC or by gas chromatography-mass spectroscopy (GC-MS) analyses. GLC analyses were performed using a HP 6890 instrument equipped with FID detector and a HP-1 (Crosslinked Methyl Siloxane) capillary column (60.0 m × 0.25 mm × 1.0 μm). Conversions and yields were calculated by GLC analysis using biphenyl as the internal standard. GC-MS data (EI, 70 eV) were acquired on a HP 6890 instrument (HP-Agilent, Santa Clara, CA, USA) using a HP-5MS cross-linked 5% PH ME siloxane (30.0 m × 0.25 mm × 0.25 μm) capillary column coupled with a mass spectrometer HP 5973 (HP-Agilent, Santa Clara, CA, USA). The products were identified by comparison of their GC-MS features with those of authentic samples.

FT-IR spectra (in KBr pellets) were recorded on a Jasco FT/IR 4200 spectrophotometer. Thermogravimetric analyses (TGA) and simultaneous differential scanning calorimetry (DSC) measurements were performed in nitrogen flow (40 mL min⁻¹) with a TA instruments SDT Q600 thermal analyzer in the range from 30 to 1000 °C with a heating rate of 10 °C min⁻¹. Triplicate TGA and DSC runs have been performed to ensure reproducibility.

Surface morphology was investigated on a selected piece of nanocomposite considered to be representative of the material. A FESEM Zeiss Σigma 300 VP (Zeiss Oberkochen, Germany) equipped with an energy dispersive spectrometer (EDS) C-MaxN SDD (Oxford Instruments, Oxford, U.K.) with an active area of 20 mm² (Oxford Instruments, Oxford, U.K.) was used to perform analysis on the selected samples. The EDS spectrometer was calibrated using MAC standards (Micro-Analysis Consultants Ltd., United Kingdom) for elemental analysis. TEM analyses were performed with an FEI Tecnai 12 instrument (120 kV; filament: LaB6), by dropping 20 lL of 1 g/L powder suspensions in water on Formvar-coated Cu grid (300 mesh, Agar Scientific, Stansted, UK). The microscope was calibrated by using the S106 Cross Grating (Agar Scientific, Stansted, UK). Alignments and astigmatism correction were carried out following the factory settings and fast Fourier transform processing, respectively.

X-ray powder diffraction (XRD) analysis were carried out on the synthetized samples using a Miniflex II (Rigaku Corporation, Tokyo, Japan) diffractometer equipped with Cu tube (Cu-Kα) working at 15 mA and 30 kV. Since the amount of synthetized sample was very small, NaF powder (Mallinckrodt Baker B.V., Holland) was pressed inside the sample holder. Then, the synthetized sample was ground in an agate mortar, placed at the center of the NaF substrate and pressed to have a flat surface. XRD data acquisitions were carried out in the range 3–70° 2θ (step width of 0.02° 2θ, counting time 3 s/step) using a Soller slit of 0.3 mm, a divergent slit of 1.25°, a mask of 10 mm and an antiscatter slit of 1.25°. XRD patterns performed for pure NaF, CeO₂, Fe₃O₄, cerium (III) oxalate hydrate and iron (II) oxalate dihydrate (humboltite) have been reported in the references listed in Table S1.

3.2. Synthesis of Magnetite-Ceria Nanocomposites

There are two steps to synthesize the magnetically recoverable magnetite-ceria nanoparticles: the synthesis of Fe₃O₄ nanoparticles, followed by nanoceria formation.

3.2.1. Synthesis of Fe₃O₄ Nanoparticles

Magnetite nanoparticles were prepared by a hydrothermal synthesis assisted by microwave irradiation using FeCl₂·4H₂O as precursor and hydrated hydrazine in alkaline condition as oxidant agent, as reported in ref. [36]. The nano powders obtained by microwave-assisted synthesis were labeled as Fe₃O_4, when no additive was used, and as Fe₃O₄:PVP, Fe₃O₄:CITR and Fe₃O₄:OA when polyvinylpyrrolidone, trisodium citrate dihydrate or oleic acid were used as additives during synthesis, respectively.

3.2.2. Coating of Fe₃O₄ Nanoparticles with Ceria

In a typical reaction, Fe₃O₄ (0.1000 g, 0.432 mmol) and (NH₄)₂Ce(NO₃)₆ (0. 3534 g, 0.645 mmol, Ce/Fe₃O₄ molar ratio = 1.5) were mixed in ethylene glycol (15 mL) in a 100 mL Teflon® autoclave. The Teflon^® autoclave was then sealed and placed in the microwave applicator using 2.45 GHz frequency with a maximum power of 1000 W. The microwave power was adjusted to reach the settled temperature of 200 °C in 10 min with a dwell time of 45 min. The temperature was monitored by means of a direct optic fiber temperature sensor inserted in a thermowell of the reference vessel. After reaction, the vessel was cooled down to room temperature by means of ventilation applied to the vessels inside the microwave applicator. The as-obtained dark suspension was centrifuged and washed with water (3 × 5 mL) and acetone (5 mL), and a black powder was finally obtained after drying in oven at 80 °C overnight. Yield = 83%, m = 174.7 mg. This nanomaterial was labeled as Fe₃O₄/1.5CeO₂, to specify that the Ce/Fe₃O₄ molar ratio used during synthesis was 1.5.

Similarly, by employing Fe₃O₄:PVP (0.1000 g, 0.432 mmol), Fe₃O₄:CITR (0.1000 g, 0.432 mmol) and Fe₃O₄:OA (0.1000 g, 0.432 mmol), respectively as seeds instead of Fe₃O₄, and by using the above mentioned procedure, the following nanomaterials were obtained: Fe₃O₄:PVP/1.5CeO₂ (Yield = 82%, m = 172.4 mg), Fe₃O₄:CITR/1.5CeO₂ (Yield = 90%, m = 190.4 mg) and Fe₃O₄:OA/1.5CeO₂ (Yield = 60%, m = 126.7 mg).

The same procedure was repeated by using Fe₃O₄ (0.1000 g, 0.432 mmol) or Fe₃O₄:PVP (0.1000 g, 0.432 mmol) or Fe₃O₄:CITR (0.1000 g, 0.432 mmol) or Fe₃O₄:OA (0.1000 g, 0.432 mmol) in the presence of (NH₄)₂Ce(NO₃)₆ (0.1413 g, 0.258 mmol, Ce/Fe₃O₄ molar ratio = 0.6), yielding Fe₃O₄/0.6CeO₂ (Yield = 64%, m = 128.4 mg), Fe₃O₄:PVP/0.6CeO₂ (Yield = 61%, m = 122.4 mg), Fe₃O₄:CITR/0.6CeO₂ (Yield = 61%, m = 127.0 mg) and Fe₃O₄:OA/0.6CeO₂ (Yield = 29%, m = 110.6 mg), respectively.

3.3. Synthesis of Ceria Nanoparticles 1.5CeO₂ and 0.6CeO₂

For comparison purposes, pure CeO₂ nanoparticles were also prepared by the microwave-assisted solvothermal method in the same conditions reported in the previous paragraph, but in the absence of magnetite seeds, yielding 1.5CeO₂ and 0.6CeO₂, respectively. Ceria nanoparticles were, thus, prepared by a solvothermal synthesis assisted by microwave irradiation using (NH₄)₂Ce(NO₃)₆ as precursor. In the general procedure, a solution was prepared dissolving (NH₄)₂Ce(NO₃)₆ (0.3534 g, 0.645 mmol or 0.1413 g, 0.258 mmol, for the synthesis of 1.5CeO₂ or 0.6CeO₂, respectively) in ethylene glycol (15 mL) in a 100 mL Teflon^® autoclave. The same procedure described above was used for the microwave treatment.

0.6CeO₂: yield = 48%, m = 21.2 mg.

1.5CeO₂: yield = 76%, m = 84.1 mg.

3.4. Catalytic Experiments

Benzyl alcohol (21.6 mg, 0.2 mmol) and aniline (37.3 mg, 0.4 mmol) were put in a glass tube under air without solvent. Then, the appropriate amount of catalyst (containing in any case 10 mg of CeO₂) was added and stirred vigorously at 50 °C for 3h. Afterwards, the reaction mixture was cooled to room temperature and added of the internal standard (diphenyl, 10 mg) for conversion assessment and ethanol (2 mL). The catalyst was pushed to the bottom of the tube with the help of an external magnet and the liquid solution was transferred to a new vial. The catalyst was then washed with ethanol (3 × 1 mL) and the washing liquors were collected and added to the reaction solution for GLC analyses. In order to compare catalytic results with those obtained by other researchers, the reaction rates were used to assess the catalytic ability, by calculating the imine formation amount per hour per catalyst amount (mmol h⁻¹g⁻¹).

3.5. Reusing of the Catalyst

The catalyst removed from the reaction solution was dried in oven at 80 °C overnight and then calcinated at 400 °C for 1 h under air, before being employed for a subsequent catalytic run.

4. Conclusions

A microwave-assisted solvothermal procedure in ethylene glycol was proposed to prepare magnetite/ceria nanocomposites by depositing nanoceria on magnetite seeds of different morphologies (hexagonal, spheroidal, quasi-spherical), using 0.6 and 1.5 Ce/Fe₃O₄ molar ratio during synthesis. IR, XRD, TEM and TGA/DSC analyses suggested: (i) to calcinate the new materials at 550 °C for avoiding sintering processes and promoting the ceria transition phase from amorphous to crystalline state; and (ii) to use at least 1.5 Ce/Fe₃O₄ molar ratio during preparation for obtaining a more stable magnetite/ceria nanostructure. Thus, the new nanocomposites synthetized with 1.5 Ce/Fe₃O₄ molar ratio and calcined at 550 °C were carefully investigated by IR, XRD, FESEM, EDS and TEM analyses. The Fe₃O₄ nanoparticles kept their pristine morphology even after ceria deposition and subsequent calcination at 550 °C. All nanocomposites resulted magnetically active, thus easily removable from the reaction mixture with the help of an external magnet. The synthetized nanomaterials were tested as catalysts for imine formation from benzyl alcohol and aniline in the absence of any solvent under air. The highest imine conversion rate was obtained with Fe₃O₄/CeO_2, prepared in the absence of any organic stabilizer. Its catalytic activity was even higher than commercial nanoceria. The new catalyst was recyclable for at least five runs. Its catalytic activity increased with the reuses probably because the organic impurities coming from the solvothermal synthesis depleted over the runs, as suggested by XRD analyses, rendering the whole system highly interesting in the field of nano-catalysis.