Bifunctional Fe₂O₃ catalyst for the hydrogenation and transfer hydrogenation of nitroarenes

Abstract

Fe₂O₃-200, prepared via a facile precipitation method, could activate H₂ and stoichiometric N₂H₄·H₂O to reduce nitrobenzene, thus forming aniline. Fe₂O₃-200 could efficiently adsorb and activate the nitro group, promoting the hydrogenation of nitroarenes. Notably, the decomposition of N₂H₄·H₂O relies on the presence of the nitro group. Moreover, Fe₂O₃-200 exhibits good stability and universality.

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Aryl amines, pivotal as organic intermediates and raw materials, play a critical role in the industrial synthesis of products including fine chemicals, dyestuffs, agrochemicals and pharmaceuticals.¹ The reduction of nitroarenes, a key step in producing these amines, has seen the exploration of various catalysts, such as noble metals,² non-noble metals,³ and metal-free⁴ compounds. Nowadays, there is still wide interest in developing an efficient and low-cost catalyst for aryl amine synthesis from nitroarenes.

Iron, being one of the most abundant elements on Earth, plays a significant role in heterogeneous reactions.⁵ In 1854, Pierre J. A. Béchamp employed iron as a reducing agent for nitrobenzene (NB) reduction in acidic media to synthesize aniline (AN).⁶ Subsequently, iron derivatives such as oxides, nitrides, and carbides have been advanced and applied in nitrobenzene reduction.⁷ In 2013, Beller et al. developed an iron-based catalyst, Fe–N/C, using iron/phenanthroline complexes on carbon for nitroarene hydrogenation with H₂, emphasizing the role of unique FeN_x centers.⁸ In 2016, Zou et al. introduced defect-engineered Fe₃O₄ for enhancing nitroarene hydrogenation using H₂.⁹ More recently, in 2023, Xiong et al. created a Fe₃O₄@TiO₂ photocatalyst using hydrazine hydrate, identifying Fe₃O₄ as the active phase.¹⁰ Despite the development of iron-based catalysts, challenges remain, particularly concerning their labor-intensive preparation processes, which are currently unsuitable for industrial-scale production. Additionally, the performance specifics of Fe₂O₃ in the hydrogenation and transfer hydrogenation of nitroarenes have not been thoroughly investigated, highlighting a critical area for future research.¹¹

Atom economy, emphasizing resource and energy efficiency, has been a compelling subject in diverse synthetic reactions for centuries.¹² In the context of transfer hydrogenation of nitrobenzene, numerous catalysts have been reported, often employing an excess of hydrazine hydrate, a practice that contradicts the principles of atom economy.¹³ Hence, stoichiometric transfer hydrogenation of nitrobenzene emerges as a vital area of research. This work builds on our previous studies on iron oxides, wherein Ce and Zr were doped into a Fe₂O₃ host to modulate its electronic structure and enhance its catalytic activity, and the resulting catalyst could efficiently catalyze nitroaromatics with stoichiometric hydrazine hydrate over extended periods. This observation, coupled with the noteworthy activity of pure Fe₂O₃ at reaction temperatures below 60 °C, inspired us to further investigate and augment the activity of Fe₂O₃.¹⁴

In this study, Fe₂O₃ series catalysts were synthesized and applied for the reduction of nitrobenzene. The detailed synthesis method and general procedure are given in ESI.† Samples not specifically emphasized are precipitated by NH₃·H₂O.

As shown in Table 1, the as-prepared samples were applied for the hydrogenation of nitrobenzene to synthesize aniline. Beyond the iron oxides prepared under varying conditions, the reactivity of other metal oxides, such as CeO₂, NiO, Cr₂O₃, La₂O₃ and Mn₈O₅, was also evaluated in this reaction (Table S1, ESI†). However, none exhibited superior activity to Fe₂O₃-200. Consequently, our findings indicate that among the tested metal oxides, iron oxide stands out as the most effective catalyst for this reaction, facilitating complete conversion of nitrobenzene at ambient temperature with a stoichiometric amount of hydrazine hydrate. To better understand the superior performance of Fe₂O₃-200, other reported catalysts using N₂H₄·H₂O as hydrogen donor are compared in the ESI† (Table S2), which indicates that this work only needs a much lower temperature and N₂H₄·H₂O dosage than most non-noble metal catalysts, even lower than some of the noble metal catalysts listed in Table S2 (ESI†). Moreover, the turnover number (TON) and turnover frequency (TOF) of Fe₂O₃-200 in the reduction of nitroarenes were 3.91 and 1.96 h⁻¹, respectively.

Table 1 Catalytic performance for the transfer hydrogenation of nitrobenzene

Entry	Catalysts	Con. [%]	AN yield [%]
Conditions: 1.5 mmol N2H4·H2O, 1 mmol nitrobenzene (NB), 50 mg catalysts, 2 mL EtOH, 2 h, 800 rpm. Determined through gas chromatography (GC) using n-hexadecane as the internal standard.
1	Fe2O3-60	5	5
2	Fe2O3-200	100	100
3	Fe2O3-300	12	5
4	Fe2O3-200–NaOH	13	13
5	Fe2O3-200–Na2CO3	51	51
6	Fe2O3-200–KOH	32	32
7	Fe2O3-200–K2CO3	1	1

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The XRD patterns, presented in Fig. S1 (ESI†), show that samples calcined at 200 and 60 °C display almost no detectable diffractions. Furthermore, SEM images of the Fe₂O₃-x samples, included in the ESI† (Fig. S2), reveal no notable differences. The N₂ adsorption–desorption profiles of these samples, depicted in Fig. S3 (ESI†), provide insights into their physical properties. All samples exhibit type IV isotherms and type H2 hysteresis loops, characteristic of mesoporous materials as per the IUPAC classification.¹⁵ The data concerning the surface area and pore volume are also provided in Table S3 (ESI†). Fe₂O₃-60 has the highest surface area, and this decreases with increasing calcination temperature. Despite its high surface area, Fe₂O₃-60 did not demonstrate exceptional catalytic performance (Table 1), with no significant correlation between the surface area and catalytic activity. Overall, the catalytic activity of Fe₂O₃ is influenced by a combination of various factors. The Fe 2p XPS results shown in Fig. S4 (ESI†) of the sample also indicate that the calcination temperature has no significant effect on the valence state of iron.

Based on our previous works on nitroarene hydrogenation, NH₃-TPD has emerged as a crucial and effective method for assessing hydrazine activation activity, with the results presented in Fig. 1. Fe₂O₃-60 possessed the highest desorption peaks of NH₃, which decreased as the calcination temperature increased. As the dehydration during the heating process cannot be distinguished, we employed an acid titration method. This involved mixing a measured amount of N₂H₄·H₂O with growing quantities of catalysts (refer to ESI† for detailed experimental procedures). The pH approached neutrality upon adding 100 mg Fe₂O₃-200, and the pH reduction rate was the fastest for Fe₂O₃-200 compared to Fe₂O₃-60 and Fe₂O₃-300. This indicates that Fe₂O₃-200 has superior N₂H₄·H₂O adsorption capacity, enhancing its ability to catalyze the conversion of nitrobenzene to aniline more efficiently and effectively.

Fig. 1 NH₃-TPD (a) results and acid titration test results (b) of different samples.

Importantly, N₂H₄·H₂O would produce N₂ and H₂O in the reaction, with bubble formation being a key indicator of this decomposition. Notably, no bubbles were observed during the acid titration tests mentioned earlier, prompting further investigation into the mechanism of hydrazine decomposition.¹⁶ Consequently, we conducted experiments by depositing all reactants separately in a round-bottom tube to ascertain the specific conditions for hydrazine decomposition. Magnetic stirring was temporarily halted to allow for clear observation and documentation of the reaction states, as depicted in Fig. 2. Bubbles were observed only in the setup shown in Fig. 2(a), which contained nitrobenzene, Fe₂O₃-200, and N₂H₄·H₂O. Absence of either nitrobenzene or the catalyst halted the decomposition of N₂H₄·H₂O. Thus, it can be concluded that both the catalyst and nitrobenzene play crucial roles in facilitating the decomposition of N₂H₄·H₂O.

Fig. 2 Photography of ethanol, NB, Fe₂O₃-200 and N₂H₄·H₂O (a), ethanol, Fe₂O₃-200 and N₂H₄·H₂O (b), ethanol, Fe₂O₃-200 and NB (c), ethanol and N₂H₄·H₂O (d), ethanol and NB (e).

To elucidate the underlying interactions among NB, the catalysts, and hydrazine hydrate, Fourier-transform infrared spectroscopy (FT-IR) analysis was conducted, with the results presented in Fig. 3. The N–H bending vibration peak of hydrazine is identified at 1620 cm⁻¹.¹⁷ Notably, curve III in Fig. 3 shows a shift compared to pure N₂H₄·H₂O (curve II). After adding nitrobenzene to the system (curve III), the N–H bond peak decreases, indicating the decomposition of N₂H₄. In curve III–VI, there are two stretching vibrations of the N–O band. One is the asymmetric vibration at 1528 cm⁻¹, and the other is the symmetric vibration at 1349 cm⁻¹.¹⁸ Upon interaction with N₂H₄, a slight shift towards lower vibration frequencies in these N–O peaks in curve III was observed, indicating the activation of the nitro group in nitrobenzene and its involvement in the N₂H₄ decomposition process.

Fig. 3 FT-IR patterns for illustrating the interactions among Fe₂O₃-200, nitrobenzene and hydrazine hydrate. (I) N₂H₄·H₂O; (II) N₂H₄·H₂O and Fe₂O₃-200; (III) N₂H₄·H₂O, nitrobenzene and Fe₂O₃-200; (IV) N₂H₄·H₂O and nitrobenzene; (V) nitrobenzene; (VI) nitrobenzene and Fe₂O₃-200.

Furthermore, to decipher the reaction pathway in the transfer hydrogenation of nitrobenzene catalyzed by Fe₂O₃-200, high-performance liquid chromatography (HPLC) was employed to identify the intermediate products formed during the reaction, as depicted in Fig. 4(a). The only intermediate detected was N-phenylhydroxylamine (PHA), whose concentration initially increased and subsequently declined, aligning with typical intermediate product behaviour. Consequently, the hydrogenation pathway of NB in our system can be summarized as follows: Ph-NO₂ → Ph-NHOH → Ph-NH₂. Notably, other potential intermediates (such as Ph-NO, azoxybenzene, and azobenzene) were not observed in our reaction system.

Fig. 4 (a) The kinetic experimental result of NB. Reaction conditions: NB (1 mmol), N₂H₄·H₂O (1.5 mmol), 50 mg Fe₂O₃-200, 2 mL EtOH as solvent, determined using HPLC. (b) and (c) The XPS results of N 1s and Fe 2p, (I) Fe₂O₃-200; (II) Fe₂O₃-200 and N₂H₄·H₂O; (III) Fe₂O₃-200, nitrobenzene and N₂H₄·H₂O. (d) Nitro-assisted hydrazine decomposition mechanism.

The substrate adsorption XPS experiment was conducted to examine the interaction between the catalyst and N₂H₄·H₂O, and the detailed process is shown in the ESI.† In the N 1s spectrum (Fig. 4b), sample II, which mixed the catalyst and N₂H₄·H₂O, exhibited two split peaks, with the peak around 400 eV identified in the literature as a metal–N peak.¹⁹ In the Fe 2p spectrum (Fig. 4c), compared to Fe₂O₃-200 (sample I), sample II showed a 0.7 eV higher peak, confirming the formation of Fe–N bonds in sample II. In contrast, sample III, which mixed N₂H₄·H₂O, NB and Fe₂O₃-200, displayed no significant peak within the detection interval of N, and the shift in the Fe 2p curve disappeared, indicating the cleavage of the Fe–N bond after the reaction. These results provide insight into the interaction between N₂H₄·H₂O and Fe₂O₃-200, with this precise reaction behaviour explaining the high utilization rate of N₂H₄·H₂O in the hydrogenation of NB.

Drawing from the comprehensive experimental results and observed characteristics, we propose a plausible hydrazine decomposition mechanism, as illustrated in Fig. 4(d). Acid titration test results indicated that the acid sites of Fe₂O₃-200 play an important role in the hydrogenation of nitrobenzene with N₂H₄·H₂O. During the reaction, the nitrogen atom in hydrazine first binds to the acidic sites on iron oxide. The nitro group then participates in the reaction, with the oxygen atom in the nitro group forming a hydrogen bond with hydrogen in hydrazine, weakening the N–H bond. This interaction leads to the release of a water molecule, followed by the adsorption of the remaining hydrazine moiety on the catalyst, facilitating the hydrogenation process to yield aniline.

Surprisingly, when changing the reducing agent to hydrogen gas, Fe₂O₃-200 also possessed the ability to activate H₂ for the reduction of nitrobenzene, as shown in Table 2. Fe₂O₃-300 could hardly activate hydrogen. H₂-TPR results, shown in Fig. 5a, revealed that these two Fe₂O₃ samples exhibited two distinct reduction peaks at around 356 and 570 °C. This suggests a stepwise reduction progress of Fe₂O₃ to Fe, with Fe₃O₄ and FeO as intermediates (e.g., Fe₂O₃ → Fe₃O₄ → FeO → Fe⁰). Despite the lack of significant differences in the H₂ activation capabilities among these samples, further investigation was conducted using alternative characterization techniques.

Table 2 Catalytic performance for the transfer hydrogenation of nitrobenzene

Entry	Catalysts	Con. [%]	AN yield [%]
Conditions: 1 mmol NB, 100 mg catalysts, 5 mL n-hexane and 5 mL isopropanol as solvent, 150 °C, 1 MPa H2, 16 h, 800 rpm. Determined using GC using n-hexadecane as the internal standard.
1	Fe2O3-200	95	93
2	Fe2O3-300	70	50
3	Fe2O3-200–NaOH	9	5
4	Fe2O3-200–Na2CO3	99	68
5	Fe2O3-200–KOH	96	60
6	Fe2O3-200–K2CO3	18	2

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Fig. 5 H₂-TPR results of the as-prepared samples (a), and FT-IR patterns of only NB and of NB adsorbed on different catalysts (b).

In Fig. 5b, the characteristic peaks of the N–O bond, located at 1528 cm⁻¹ and 1349 cm⁻¹, shifted to lower wavenumbers after adsorbing onto the catalysts. Notably, nitrobenzene adsorbed on Fe₂O₃-200 exhibited the maximum shift, suggesting the strongest interaction between the catalyst and the substrate. This correlates with the highest activity of Fe₂O₃-200 in the presence of H₂.

The results of the catalyst recycling experiments, as depicted in Fig. S5(a) and S6(a) (ESI†), demonstrated that the catalyst maintained high conversion rates over five consecutive cycles, irrespective of the use of N₂H₄·H₂O or H₂ as the reducing agent. Furthermore, the hot filtration experiments, shown in Fig. S5(b) and S6(b) (ESI†), provided evidence of the catalyst's stability. These experiments indicated that the reaction ceased immediately after the catalyst was removed from the reaction system, confirming the crucial role of the catalyst in sustaining the reaction process.

The scope of Fe₂O₃-200 in the reduction of nitroarenes to the corresponding anilines was also investigated, and the results are listed in Table 3. The results demonstrate that Fe₂O₃-200 effectively reduces nitroarenes to anilines, achieving excellent yields under both reducing conditions N₂H₄·H₂O and H₂. Importantly, this catalyst shows high efficiency regardless of whether the nitroarene substrates contain electron-donating or electron-withdrawing groups, catalyzing the formation of the corresponding anilines with high yields. This highlights the robust catalytic activity of Fe₂O₃-200 across various reaction environments.

Table 3 The substrate scope of Fe₂O₃-200

Conditions: (a) 1.5 mmol N₂H₄·H₂O, 1 mmol substrate, 50 mg catalysts, 2 mL EtOH, 800 rpm. (b) 1 mmol substrate, 100 mg catalysts, 5 mL n-hexane and 5 mL isopropanol as solvent, 150 °C, 1 MPa H₂, 800 rpm. Determined by gas chromatography-mass spectrometry (GC-MS) and GC using n-hexadecane as the internal standard (time (h)/conversion/selectivity).

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In conclusion, the Fe₂O₃-200 synthesized in our study is capable of activating both H₂ and N₂H₄·H₂O for the reduction of nitro compounds to anilines. NH₃-TPD and acid titration tests confirmed the substantial presence of acidic sites on the surface of Fe₂O₃-200, which are instrumental in the adsorption and decomposition of hydrazine hydrate. This feature also contributes to the catalyst's efficiency in the transfer hydrogenation of nitroarenes. The experimental observations, coupled with FT-IR results, support a nitro-assisted hydrazine decomposition mechanism. The demonstrated –N–O activation ability of Fe₂O₃-200 ensures its effectiveness in both hydrogenation and transfer hydrogenation processes.

Data availability

All data generated in this study are provided in the manuscript and its ESI.† The other data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22278199), Gansu Association for Science and Technology: Innovation Driven Assistance Engineering Project (GXH20230817-1, GXH20230817-16), the Fundamental Research Funds for the Central Universities (lzujbky-2023-ct01), and the Science and Technology Planning Project of Chengguan District in Lanzhou (2023JSCX0049).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qm00605d

‡ These authors contributed equally to this work.

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