Facile synthesis of FeCeOx nanoparticles encapsulated carbon nitride catalyst for highly efficient and recyclable synthesis of substituted imidazoles

Herein, we developed a novel composite called FeCeOx@g-C3N4 through a combination of sonication, sintering, and hydrothermal techniques to implement the principles of green chemistry by utilizing reusable nanocomposites in one-pot reactions. To gain a comprehensive understanding of the catalyst's structure, composition, and morphology, various characterization methods were employed. These included FT-IR analysis to examine chemical bonds, SEM and TEM imaging to visualize the catalyst's surface and internal structure, TGA to assess thermal stability, EDS for elemental composition analysis, and XRD to determine crystal structure. The FeCeOx@g-C3N4 nanocatalyst demonstrated remarkable efficacy in the one-pot synthesis of 2,4,5-trisubstituted and 1,2,4,5-tetrasubstituted imidazole. Noteworthy features of this catalyst included high percentage yield, mild reaction conditions, short reaction time, and an efficient and straightforward procedure. Furthermore, the FeCeOx@g-C3N4 composite exhibited excellent recyclability and reusability. It could be recycled and reused up to four times without a significant decline in catalytic activity.

g-C 3 N 4 has two main limitations: first, it absorbs only a small fraction of solar energy, primarily in the range of low bandgap wavelengths (below 460 nm).Second, the fast recombination of double electrons within the cavities of g-C 3 N 4 leads to a decrease in photocatalytic activity [27][28][29][30] .To address these limitations, various approaches have been developed to enhance the catalytic activity of g-C 3 N 4 and mitigate imperfections 31 .These approaches include doping g-C 3 N 4 with transition metals [32][33][34][35] and coupling it with metals [36][37][38][39][40][41] .These strategies aim to improve the absorption of a broader range of solar energy, reduce electron recombination, and enable the recovery and recycling of g-C 3 N 4 , thereby enhancing its overall performance as a catalyst [42][43][44] .
Magnetic materials offer a range of advantages due to their inherent magnetism and unique properties 45,46 .They are crucial in various industries and applications 47 .Magnetic storage devices rely on their ability to retain magnetization, enabling high-capacity data storage 48 .Electric motors and generators utilize magnetic materials for efficient energy conversion 49 .Magnetic sensors enable precise detection and measurement of magnetic fields in compasses, position sensing, and current sensing 50 .In industries like mining and recycling, magnetic materials facilitate effective separation techniques 51 .Biomedical applications benefit from magnetic nanoparticles in imaging, drug delivery, and cancer treatment 52 .Magnetic materials also play a vital role in non-destructive testing and offer versatility for customization 53 .
In our continue interest to the expanding knowledge of carbon nitride-based catalysts and their application in various organic transformations [54][55][56] , herein, we successfully prepared FeCeO x @g-C 3 N 4 nanocomposites, which serve as a novel catalyst with excellent activity in the one-pot synthesis of 1,2,4,5-tetraphenylimidazole and 2,4,5-triphenylimidazole derivatives under mild reaction conditions.By combining FeCeO x with g-C 3 N 4 , we have fabricated a catalyst with enhanced performance and activity that offer practical benefits, such as energy efficiency and environmental friendliness.

SEM
The morphology and microstructure of the FeCeO x @g-C 3 N 4 nanocomposite were investigated using SEM analysis.The obtained SEM images of FeCeO x @g-C 3 N 4 clearly demonstrate the deposition of FeCeO x on the surface of g-C 3 N 4 , as shown in Fig. 1.The SEM images reveal a 2D sheet-like network structure with a uniform distribution of Fe and Ce species on the surface of g-C 3 N 4 .This indicates successful incorporation of FeCeO x onto the g-C 3 N 4 framework.Importantly, the absence of aggregated species suggests good dispersion and adherence of FeCeO x nanoparticles on the g-C 3 N 4 surface.

EDS
The chemical composition of FeCeO x @g-C 3 N 4 was further confirmed through EDS spectrum analysis.The EDS spectrum, as shown in Fig. 2, reveals the presence of elements associated with Ce, Fe, O, N, and C. The appearance of these elements in the EDS spectrum provides strong evidence for the incorporation of Ce and Fe in the nanocomposite.The presence of Ce indicates the successful integration of CeO x , while Fe confirms the presence of FeO x .This supports the earlier observations from SEM analysis, indicating the successful deposition of FeCeO x nanoparticles onto the g-C 3 N 4 framework.

XRD
The crystal structure of the composite was characterized using a powder X-ray diffractometer (XRD).As shown in Fig. 3 The detailed morphology and structure of the nanocomposite at the nanoscale level were showed in the TEM image of the FeCeO x @g-C 3 N 4 nanocomposite (Fig. 5).The image reveal FeCeOx particles had good dispersity and uniform distribution of particle size on the surface of the g-C 3 N 4 matrix.The g-C 3 N 4 matrix, on the other hand, would appear as a continuous network of interconnected sheets or layers, forming a 2D structure.The  www.nature.com/scientificreports/g-C 3 N 4 layers might exhibit a darker contrast compared to the nanoparticles, providing a contrasting background in the TEM image.Measurement of the surface area of cavities in porous materials is important.Therefore, for the synthesized FeCeO x @g-C 3 N 4 nanocomposite, the surface area, pore volumes and pore size was measured and the obtained data are depicted in Table 1 and supporting information.Specific surface area of pure g-C 3 N 4 is 41.14 m 2 g −1 .
The specific surface area of the FeCeO x /g-C 3 N 4 sample, calculated using the BET equation, is 36.12m 2 g −1 , which is lower compared to that of bare g-C 3 N 4 .The decrease of specific surface area along with lower pore volume indicates FeCeOx is loaded to g-C 3 N 4 .
Thermogravimetric analysis (TGA) is a technique used to study the thermal behavior of a material as a function of temperature.It involves measuring the weight changes of a sample as it is heated or cooled under controlled conditions (Fig. 6).The TGA of the FeCeO x @g-C 3 N 4 revealed three stages of weight loss.The initial weight loss at lower temperatures due to the removal of surface-adsorbed species.These adsorbed species can include moisture, gases, or functional groups like OH groups that may be present on the composite surface.The major weight loss occurred between approximately 520-630 °C, which was attributed to the combustion of the graphitic carbon nitride phase.www.nature.com/scientificreports/ To validate the applicability of ICP-MS as alternatives to EDX analysis for the measurement of Ce in the composite, FeCeO x @g-C 3 N 4 nanocomposite were analyzed by ICP-MS.The amount of analyzed Ce (0.71 wt.%) corresponded to Ce content (0.69 wt.%) based on EDX measurement.

Assessment of catalytic activity of FeCeO x @g-C 3 N 4 nanocomposite for the synthesis of 2,4,5-trisubstituted and 1,2,4,5-tetrasubstituted imidazoles
After preparing and characterizing FeCeO x @g-C 3 N 4 , the catalytic performance of the composites was investigated for the synthesis of 1,2,4,5-tetra phenyl imidazole and 2,4,5-triphenyl imidazole.This study aimed to develop a cost-effective and easily accessible method for synthesizing these imidazole derivatives using readily available starting materials.The catalytic protocol demonstrated excellent selectivity and simplicity, allowing for the synthesis of various 2,4,5-trisubstituted and 1,2,4,5-tetrasubstituted imidazoles.The procedure presented a sustainable and chemically efficient alternative, as it utilized inexpensive and readily available starting materials.

Optimization of the reaction parameters for the synthesis of 1,2,4,5-tetrasubstituted imidazoles
In the optimization study, benzaldehyde, benzil, aniline, and NH 4 OAc were chosen as model substrates to prepare 1,2,4,5-tetrasubstituted imidazole.The goal was to identify the optimal reaction conditions by varying catalysts, solvents, and temperatures.Table 2 presents the results of the model reactions under different conditions.Entry 1 shows that the model reaction without a catalyst resulted in a low yield, indicating the importance of a catalyst in the reaction.Subsequently, various catalysts were tested in the model reaction (entries 2-6 and 8), and the best outcome was obtained with FeCeO x @g-C 3 N 4 as the catalyst (entry 8).Further investigation was conducted to determine the optimal catalyst amount (entries 7-10).The results in Table 2 revealed that using 20 mg of FeCeO x @g-C 3 N 4 as the catalyst (entry 8) provided the highest yield for synthesizing 1,2,4,5-tetraphenyl imidazoles using benzaldehyde (0.5 mmol), benzil (0.5 mmol), aniline (0.5 mmol), and ammonium acetate (0.5 mmol).
The optimization study further investigated the effect of temperature and solvent on the synthesis of 1,2,4,5-tetrasubstituted imidazole.Table 2 provided insights into the optimal reaction conditions.Regarding temperature, the results in Table 2 (entries 8 and 11) indicated that 60 °C was the best operating temperature for the model reaction, resulting in the highest yield of the desired product (Table 2, entry 8).Moreover, the impact of different solvents on the model reaction was explored (Table 2, entries 12-19).Among the solvents tested, ethanol (EtOH) was found to be the most suitable choice for this reaction, providing favorable yields of the 1,2,4,5-tetraphenylimidazole product.Based on the results presented in Table 2, the optimized conditions for the synthesis of 1,2,4,5-tetraphenylimidazole are as follows: benzaldehyde (0.5 mmol), benzil (0.5 mmol), aniline (0.5 mmol), and ammonium acetate (0.5 mmol) as the substrates, 20 mg of FeCeOx@g-C 3 N 4 nanocomposite as the catalyst, ethanol as the solvent, and a reaction temperature of 60 °C.
The relationship between reaction time and percent yield in the model reaction was evaluated by collecting experimental data points at various reaction times.These data points were then used to plot a graph, typically referred to as Fig. 7, to visualize the trend.According to the graph, it can be observed that there is an initial increase in the percent yield as the reaction progresses.This indicates that the desired product is being formed over time, leading to an improvement in yield.However, after reaching a certain point, the graph shows a plateau, where the percent yield remains relatively constant.Beyond the plateau, a continuous increase in the yields is observed.The continuous increase in yields indicates that the reaction is proceeding in a favorable direction, leading to higher overall conversion and yield.
After optimizing the reaction conditions for synthesizing 1,2,4,5-tetraphenyl imidazole, various benzaldehydes were utilized to prepare different imidazoles.The results of these reactions are presented in Table 3, shedding light on the influence of different substituents on the benzaldehyde.The Table 3 demonstrates that both electron-withdrawing and electron-rich groups on the benzaldehyde perform well in the reaction, leading to the formation of the corresponding 1,2,4,5-tetraphenyl imidazoles in excellent yields.This suggests that a wide range of benzaldehyde derivatives can be utilized as substrates in this transformation.However, when the benzaldehyde contains an electron-donating group substituent (Table 3, entry 4), the yield of the desired product decreases.On the other hand, if the benzaldehyde possesses a strong electron-withdrawing substituent (Table 3, entry 7), the yield increases.These observations indicate that the nature of the substituents on the benzaldehyde has a significant impact on the reaction outcome.Electron-withdrawing groups tend to enhance the reactivity www.nature.com/scientificreports/and favor the formation of the desired product, leading to higher yields.Conversely, electron-donating groups may hinder the reaction progress, resulting in lower yields.

Optimization of the reaction parameters for the synthesis of 2,4,5-trisubstituted imidazoles
Based on our previous results, further optimization was carried out in the model reaction of benzaldehyde, benzil, and ammonium acetate.Reaction conditions were optimized by varying catalysts, solvents, and temperatures.The model reaction was initially carried out in the absence of the catalyst (Table 4, entry 1), leading to a low product yield.Various kinds of catalysts were used in this reaction (Table 4, entries 2-7), and the best result has been appertained to FeCeO x @g-C 3 N 4 .Finally, different amounts of FeCeO x @g-C 3 N 4 nanocomposite were used to determine their effects on the reaction in the presence of ethanol at 80 °C (Table 4, entries 7-9).The optimum amount of catalyst was 20 mg for the synthesis of 2,4,5-triphenyl imidazole in the reaction of benzaldehyde (0.5 mmol), benzil (0.5 mmol), and ammonium acetate (1.5 mmol) (Table 4, entry 9).The model reaction was investigated at different temperatures (Table 4, entries 8-10).As a result, the best choice was 80 °C as the optimal temperature in ethanol as a solvent for this reaction.Based on Table 4 inspection, it was observed that the optimum condition for the synthesis of 2,4,5-triphenyl imidazole was benzaldehyde (0.5 mmol), benzil (0.5 mmol), ammonium acetate (1.5 mmol), and 20 mg of FeCeO x @g-C 3 N 4 nanocomposite as a catalyst in ethanol as a solvent in 80 °C.
After determining the optimal conditions in the model reaction, various aldehydes, including aromatic, heteroaromatic, and aliphatic types, were employed to synthesize different imidazoles under the optimized conditions.The results of these reactions are presented in Table 5.In general, benzaldehydes with various substituents, whether electron-withdrawing or electron-donating groups, exhibited good reactivity and provided the corresponding 2,4,5-trisubstituted imidazoles in moderate to good yields.This suggests that a wide range of benzaldehydes can be utilized as substrates in this reaction, allowing for the incorporation of diverse substituents into the imidazole framework.The yields of the reactions were further influenced by the nature of the substituents on the benzaldehyde.When benzaldehydes with strong electron-withdrawing substituents like NO 2 were used (Table 5, entries 7 and 9), the reaction yields were increased.This indicates that electron-withdrawing groups enhance the reactivity and favor the formation of the desired products, leading to higher yields.However, when aliphatic aldehydes were employed (Table 5, entry 11), the yields of the corresponding imidazoles were low.This suggests that aliphatic aldehydes may not be as suitable for this transformation under the given optimized conditions.www.nature.com/scientificreports/

Recycle experiments
Recyclability is an important aspect of catalysts in terms of green chemistry, and experiments were conducted on a larger scale (2.5 mol) to reduce system errors and evaluate the catalyst's reusability.In this case, 100 mg of the FeCeO x @g-C 3 N 4 nanocomposite catalyst was used for 5 mol of starting materials.After the completion of the reaction, 10 mL of ethyl acetate was added to the reaction mixture, and the catalyst was separated from the mixture using centrifugation.The separated catalyst was then washed with ethyl acetate.The washed catalyst was successfully reused for four consecutive runs of reactions without any significant decrease in reaction yields.As shown in Fig. 8, the yields of the four runs for the synthesis of 2,4,5-trisubstituted imidazoles (red column) were 98%, 97%, 97%, and 95%.Similarly, the yields for the four runs of synthesis of 1,2,4,5-tetrasubstituted imidazoles (blue column) were 98%, 97%, 95%, and 94%, respectively.The amount of catalyst remaining after the five runs was 93 mg, indicating a minor loss of catalyst during the recycling process.
To further confirm the stability of the FeCeO x @g-C 3 N 4 nanocomposite, TGA analysis and FTIR spectroscopy were performed on both the fresh and recycled catalyst.The results from TGA analysis and FTIR spectra showed no appreciable changes in the chemical structure of the recycled FeCeO x @g-C 3 N 4 after five cycles, further indicating its stability and suitability for reuse (Supporting information).
Table 6 provides a comparison of the catalytic efficiency of different methods reported in the literature for the synthesis of tetrasubstituted imidazoles.The FeCeO x @g-C 3 N 4 catalyst is specifically evaluated in the model reaction and compared to other catalysts used in similar reactions.The FeCeO x @g-C 3 N 4 catalyst demonstrated excellent activity and outperformed the other catalysts in the set (58-60,66,68).   www.nature.com/scientificreports/A reasonable mechanism for the synthesis of trisubstituted imidazoles using the FeCeO x @g-C 3 N 4 nanocomposite as a catalyst is illustrated in Fig. 9.
The mechanism can be described as follows: The reaction begins with the condensation of an aldehyde and ammonium acetate in the presence of FeCeO x @g-C 3 N 4 catalyst.The catalyst facilitates the formation of an imine intermediate through the nucleophilic addition of the amine group of ammonium acetate to the carbonyl group of the aldehyde.The imine intermediate then undergoes a subsequent reaction with a benzyl compound.This reaction can involve the nucleophilic attack of the nitrogen atom in the imine intermediate on the electrophilic carbon atom of the benzyl compound.Following the nucleophilic attack, a rearrangement occurs, leading to the formation of a trisubstituted imidazole.This rearrangement step involves the migration of substituents within the intermediate, resulting in the desired trisubstituted imidazole product.It is important to note that cerium oxide (CeO 2 ) plays a crucial role in this transformation.The presence of cerium oxide in the FeCeO x @g-C 3 N 4 nanocomposite likely enhances the catalytic activity and stability of the catalyst.Additionally, the FeCeO x @g-C 3 N 4 nanocomposite exhibits a synergistic effect, leading to increased yields of the trisubstituted imidazole product (Fig. 9).

Conclusion
In summary, the FeCeO x @g-C 3 N 4 nanocomposite is synthesized by calcinating melamine and immobilizing Ce(III) and Fe(III) on graphitic carbon nitride (g-C 3 N 4 ).This iron-based nanocomposite, in combination with cerium functionality and the good surface area of g-C 3 N 4 , shows great potential for one-pot preparations of imidazole derivatives, resulting in good to excellent yields within short reaction times.The heterogeneous catalyst is easily separated and can be reused in subsequent reactions.This nanocomposite offers several advantages as a catalyst for imidazole synthesis, including its efficient performance, easy separation, and recyclability.

Experimental Materials and chemicals
Melting points were measured in open capillaries with the Buchi 535 melting-point apparatus.The reactions were monitored by thin-layer chromatography (TLC) with UV light as detecting agents.EDS spectra and Scanning Electron Microscope (SEM) images were prepared via the TESCAN Vega3 Model.Powder X-ray diffraction (XRD) analyses were given in a Bruker AXS-D 8 Advance diffractometer.Fourier transfer infrared spectroscopy (FT-IR) in Shimadzu IR-460. 1 H NMR spectra were recorded on a 500 MHz spectrometer and 13 C NMR spectra on a 125 MHz NMR spectrometer, respectively, using CDCl3 or DMSO(D6) as a solvent; chemical shifts have been expressed in ppm downfield from TMS.

Synthesis of g-C 3 N 4
According to our previous paper, the bulk g-C 3 N 4 was synthesized by directly heating melamine in air methods 43,44 .10 g of melamine powder was placed in a covered 50 mL alumina crucible and then heated in a muffle furnace at a ramp rate of 5 °C/min and kept for three h at 550 °C in air.After cooling to room temperature, a light yellow powder was collected and stored for further use.The g-C 3 N 4 nanosheets are prepared by thermal exfoliation in the air.In detail, 2 g of bulk g-C 3 N 4 was put into an uncovered crucible for heat treatment at 550 °C for 3 h to obtain white powder.

Synthesis of FeCeO x @g-C 3 N 4 nanocomposite
At first, the g-C 3 N 4 (400 mg), which was synthesized in the previous step, was placed in a 200 mL Erlenmeyer and then dispersed in 100 mL of methanol/water (1:1) using sonication for 10 min at room temperature.Then, 40 mg CeCl 3 and 40 mg FeCl 3 was dispersed in methanol, and 20 mL NaOH (2 M) was added to the mixture and sonicated for 10 min.Henceforward, the mixture was stirred for 5 h at 60 °C, and the mixture was filtered and washed with 10 mL of methanol and dried overnight at 60 °C under vacuum to obtain FeCeO x @g-C 3 N 4.

General procedure for synthesizing 1,2,4,5-tetraphenyl imidazole
To a mixture of aniline (0.5 mmol), benzaldehyde (0.5 mmol), benzil (0.5 mmol) and ammonium acetate (0.5 mmol), FeCeO x @g-C 3 N 4 (20 mg) as catalyst and ethanol (1 mL) as a solvent were added in a 5 mL roundbottomed flask respectively.The reaction mixture was stirred with a stirrer at 60 °C for 2 h, and TLC monitored the progress of the reaction.After the reaction was completed, 20 mL of ethyl acetate was added, the catalyst was removed by centrifuge, and the catalyst was washed with ethyl acetate and reused for the subsequent reactions.The organic residue was recrystallized to obtain 1,2,4,5-tetraphenyl imidazole and derivatives as pure products.

General procedure for the synthesis of 2,4,5-triphenyl imidazole
A mixture of benzaldehyde (0.5 mmol), benzil (0.5 mmol), ammonium acetate (1.5 mmol), FeCeO x @g-C 3 N 4 (20 mg) as a catalyst, and ethanol (1 mL) as a solvent were placed in a 5 mL round-bottomed flask, respectively.The reaction mixture was stirred with a stirrer at 80 °C for 100 min, and TLC monitored the progress of the reaction.After the reaction was completed, 10 mL of ethyl acetate was added, and the catalyst was removed by centrifuge.Then, the catalyst was washed with hot ethanol and ethyl acetate and reused for the subsequent reactions.The ethanolic residue was recrystallized to obtain 2,4,5-triphenyl imidazole and derivatives as pure products.
, five reflections are observed in the XRD pattern of Fe 2 O 3 26.9°,35.42°, 43.3°, 56.1° and 61.3° that belong to the (2 2 0), (3 1 1), (4 0 0), (5 1 1) and (4 4 0) plane diffractions of Fe 2 O 3 .In addition to the observed reflection at (3 1 1), (2 2 0), (2 0 0), and (1 1 1) belongs to CeO 2 .According to the typical characteristic diffraction peaks of g-C 3 N 4 , two characteristic diffraction peaks can be found at 13.1° and 27.4°.FT-IR FT-IR spectroscopy was employed to investigate the bonding states of the FeCeO x @g-C 3 N 4 nanocomposite.The FT-IR analysis confirms the presence of FeCeO x on g-C 3 N 4 .In Fig. 4, several absorption bands are observed, each corresponding to specific bonding vibrations.The strong absorption band at 580 cm −1 is attributed to the Fe-O stretching mode, providing evidence for the presence of FeO in the nanocomposite.Additionally, the absorption bands at 740 cm −1 and 1416 cm −1 are assigned to the Ce-O stretching vibrations, further confirming the presence of CeO 2 .The absorption band at 804 cm −1 corresponds to the bending vibration of the s-triazine ring in g-C 3 N 4 , indicating the presence of g-C 3 N 4 in the nanocomposite.The absorption bands in the range of 1200-1400 cm −1 are attributed to the C-N stretching vibration mode present in g-C 3 N 4 .Furthermore, the absorption band at 1637 cm −1 corresponds to the C=N stretching vibration mode, providing further evidence of the presence of g-C 3 N 4 in the nanocomposite.The broad absorption band observed in the range of 2800-3500 cm −1 is indicative of the N-H stretching vibrations of amine groups in g-C 3 N 4 and the O-H stretching vibrations of absorbed water from the environment.

Figure 8 .
Figure 8. Recyclability of FeCeO x @g-C 3 N 4 nanocatalyst in the preparation of tri substituted imidazole (red chart) and four substituted imidazole (blue chart).

Table 4 .
The effect of various parameters on the synthesis of 2,4,5-triphenyl imidazole.a Isolated yields.

Table 6 .
Comparison of the catalytic efficiency of various catalyst in the literature.Proposed mechanism of the synthesis of trisubstituted imidazoles.