Azoxybenzene is a high-value compound with wide applications in chemical industries as dyes, pigments and agrochemicals [1, 2]. Azoxybenzene is commonly obtained by oxidation of aniline or reduction of nitrobenzene as an intermediate [3, 4]. The conventional production of azoxybenzene entails condensation of stoichiometric nitrosobenzene and hydroxylamine [5]. This process lacks of atom-economy and produces by-products which cause pollution troubles. To simply the reaction steps and prevent the production of hazardous by-products, the development of controlling the selectivity in the midst of nitrobenzene reduction to produce azoxybenzene is of ongoing research interest.

Recently, there are some developments reported to achieve satisfying yields of azoxybenzene in the presence of bases such as hydrazine hydrate or NaOH by transferring the reduction route of intermediate formed from nitrobenzene hydrogenation [6-8]. However, the green synthesis of azoxybenzene under the base-free condition is still a challenge. Another interesting finding is that azoxybenzene could be produced under the base-free condition catalyzed by Ni@C+CeO₂ [9]. In this research, the combination of Ni@C and CeO₂ exhibited the best catalytic performance to synthesize azoxybenzene compared with combinations of Ni@C with metal oxides such as Al₂O₃, TiO₂, SiO₂, which indicated that CeO₂ was crucial for the high yield of azoxybenzene. However, the structure-activity relationship of CeO₂ catalysts remain to be explored. For CeO₂ based catalysts, the cyclic alternation between Ce³⁺ and Ce⁴⁺ and the oxygen storage capacity (OSC) are regarded as the key ingredients in reactions such as hydrogenation [10], HgO oxidation [11], hydrogen generation [12], CO oxidation [13, 14] and so on. As shown in the report of azobenzene synthesis by reducing nitrobenzene, the BET surface areas of Au/CeO₂ catalysts, which were related to the oxygen storage capacity, exhibited a correlation with the catalytic activities of reducing azoxybenzene to azobenzene [15]. However, the key effect factor for reducing nitrobenzene to azoxybenzene catalyzed by CeO₂ based catalysts has not been proposed.

Considering that the morphology engineering of CeO₂ is of great importance to control their surface chemistry [16-18], herein, we synthesized a series of CeO₂ samples with seven morphologies and applied them in reducing nitrobenzene to azoxybenzene. The relationship of physico-chemical properties of the seven CeO₂ catalysts and their catalytic performance was explored to improve azoxybenzene yield under the base-free condition.

In this work, the Rod-CeO₂, B-Cube-CeO₂ and S-Cube-CeO₂ samples were prepared via hydrothermal methods. Particle-CeO₂ was synthesized by precipitation method. Meso-CeO₂ was prepared by hard-templating approach using KIT-6 as the hard template. Moreover, L-Particle-CeO₂ was prepared by calcination of Ce(NO₃)₃·6H₂O. The detailed synthesis conditions of above catalysts are listed in Supporting information. Commercial-CeO₂ (Sinopharm Chemical Reagent Co. Ltd., 11.82 m²/g) was purchased and used for comparison. In a typical procedure, 0.5 mmol nitrobenzene, 5 mL anhydrous toluene and a certain amount of catalysts were added in a 25 mL Parr autoclave equipped with the mechanical agitation. The autoclave was purged by N₂ for several times and sealed with 1MPa pressure CO, followed by stirring at 1000 rpm at 150 ℃. The products were separated by centrifugation and confirmed by Agilent 7890B-5977A GC-MS. The conversion and selectivity were determined with oxylene as an internal standard.

To probe the morphological characteristics of the seven CeO₂ samples, scanning electron microscope (SEM) and transmission electron microscopy (TEM) were carried out and shown in Fig. 1. According to the images observed in Figs. 1a and b, Rod-CeO₂ exhibits a width of 9−18 nm and a length of 70−320 nm. Figs. 1c-f depict that L-Particle-CeO₂ and Particle-CeO₂ are irregular particles with diameters of 5−20 nm and 5−18 nm respectively, while L-Particle-CeO₂ is longer than Particle-CeO₂ observed in TEM images. Synthesized by the hard-templating approach with mesoporous silica as templates, the images shown in Figs. 1g and h demonstrate that the well-ordered structures and mesoporous channels of Meso-CeO₂ samples were corresponding to the replicated KIT-6. From Figs. 1i-l, S-Cube-CeO₂ and B-Cube-CeO₂ show regular cubic shapes with varying size distributions of 7−32 nm and 16–140 nm respectively. Used for comparison, Commercial-CeO₂ samples shown in Figs. S1a and b (Supporting information) present irregular configuration with diameters between 7 nm and 42 nm.

To obtain the crystal sizes and identify the crystal phases, X-ray diffraction (XRD) was carried out with the results presented in Fig. S2 (Supporting information). All the diffraction peaks could be indexed to the typical fluorite structure of pure CeO₂. Related to the crystallinity, the width of the peaks can be used to estimate the mean crystallite size of CeO₂ based on the Scherrer's equation. From Table 1, the mean crystallite sizes follow the order B-Cube-CeO₂ > Commercial-CeO₂ > S-Cube-CeO₂ > Rod-CeO₂ > Particle-CeO₂ > L-Particle-CeO₂ > Meso-CeO₂. The sharp peaks of B-Cube-CeO₂ and Commercial-CeO₂ exhibit high intensities, suggesting their large grain sizes and well-developed crystallinity. The weakest peak of Meso-CeO₂ indicates that with poor crystallinity, there exist lattice defects in the Meso-CeO₂ [19]. The existence of these lattice defects may affect the catalytic selectivity of azoxybenzene because it was confirmed to enhance the reduction of azoxybenzene to azobenzene according to reports [15].

To further explore the morphological characteristics, the surface areas of the series of CeO₂ catalysts were measured by the BET analysis and listed in Table 1. Surface areas of these catalysts exhibit a wide range of 5.48–125.97 m²/g. For most CeO₂ samples, a smaller crystallite size contributes to a larger surface area. However, both the crystallite size and surface area of Rod-CeO₂ are larger than that of L-Particle-CeO₂ and Particle-CeO₂, which is probably due to the agglomeration of crystallites during the synthesis processes of L-Particle-CeO₂ and Particle-CeO₂ [20]. For CeO₂ catalysts with different morphologies, the larger surface area commonly contributes to higher catalytic activities.

X-ray photoelectron spectroscopy (XPS) technique was carried out to investigate the surface atomic environment and the amounts of Ce valences for the seven CeO₂ catalysts. Fig. S3 (Supporting information) shows the Ce 3d core-level spectra of these CeO₂ catalysts. In accordance with spin-orbit doublets of 3d_5/2 (V) and 3d_3/2 (U), all the multiplets can be fitted with ten peaks, labeled as U' (901.9 eV), U⁰ (898.6 eV), V' (883.9 eV), V⁰ (880.6 eV), U"' (916.1 eV), U" (906.9 eV), U (900.3 eV), V"' (897.6 eV), V" (888.5 eV) and V (881.9 eV) [21]. The main three 3d_5/2 features (V, V", V"') and the main three 3d_3/2 features (U, U", U"') are assigned to Ce⁴⁺ state. Moreover, Ce³⁺ state consists of all the satellites of 3d_5/2 and 3d_3/2 features, including V⁰, V', U⁰ and U'. Calculated by the relative integrated areas of Ce³⁺ and all the Ce 3d region, the amount of surface Ce³⁺ of CeO₂ samples are determined and listed in Table 1. The surface Ce³⁺ proportions follow the order Rod-CeO₂ > L-Particle-CeO₂ > Particle-CeO₂ > Meso-CeO₂ > S-Cube-CeO₂ > B-Cube-CeO₂ > Commercial-CeO₂. Considering the different redox ability of CeO₂ catalysts with various morphologies due to their different cyclic alternation ability between Ce³⁺ and Ce⁴⁺, there possibly be a relationship of surface Ce³⁺ and catalytic activity for the series of CeO₂ catalysts.

The catalytic activities of the series of CeO₂ catalysts for reducing nitrobenzene to azoxybenzene were tested and listed in Table 1. It suggests that Ce³⁺ sites and surface areas of CeO₂ catalysts with various morphologies affect the conversion and selectivity of azoxybenzene respectively. From Table 1, appreciable reduction of nitrobenzene could be delivered by all catalysts described above, with azoxybenzene as the predominant product and azobenzene as the by-product. The Rod-CeO₂ exhibits the best catalytic performance, with the azoxybenzene yield which is 5.6 times higher than that of the Commercial-CeO₂. The catalysts of entries 1–4 exhibit similar selectivity of azoxybenzene. The 100% selectivity of azoxybenzene by B-Cube-CeO₂ and Commercial-CeO₂ are in accordance with their relatively lower conversion. However, with 30.7% nitrobenzene conversion, Meso-CeO₂ shows only 77.1% selectivity of azoxybenzene with a large amount of azobenzene as the by-product. In many reports, the defects in catalysts could affect their catalytic performance [22, 23]. According to the XRD and BET results, the existence of lattice defects in Meso-CeO₂ may result in the high activity of the by-product azobenzene. Moreover, compared with the catalytic activity which was tested under N₂ atmosphere and listed in Table S1 (Supporting information), the reductant CO is proved to improve the azoxybenzene yield, which is due to the role of CO on improving the redox ability of CeO₂ catalysts.

According to reports, there is a relative relationship between the support surface area of ceria-based Au catalysts and their catalytic performance for reducing nitrobenzene to azoxybenzene [15]. However, in this work, Meso-CeO₂ samples with the highest surface area exhibit only 23.7% yield of azoxybenzene, which indicates that surface area of ceria catalysts is not the most important factor for the reaction of reducing nitrobenzene to azoxybenzene. By inspection of Fig. 2, it is worth noting that the yields of azoxybenzene are highly dependent on the surface Ce³⁺ proportion for all CeO₂ catalysts, which suggests that surface Ce³⁺ proportion is the key parameter for azoxybenzene synthesis. Therefore, the improvement of surface Ce³⁺ proportion is of great importance to obtain a high yield of azoxybenzene.

To obtain the highest yield of azoxybenzene catalyzed by the Rod-CeO₂, the variation reaction was carried out and illustrated in Fig. 3 with the reaction condition (nitrobenzene (0.5 mmol), catalyst (250 mg), CO (1 MPa), anhydrous toluene (5 mL), 150 ℃). The increasing trend with the reaction time could be observed in the conversion of nitrobenzene and the yield of azoxybenzene. After the 7 h reaction, the Rod-CeO₂ catalyst performed completely conversion of nitrobenzene and 89.8% azoxybenzene yield. Notably, only a certain amount of azobenzene as a by-product was formed during all the reactions.

To gain further insight into the reason of azoxybenzene with high selectivity as the main product, the preliminary mechanistic analysis was performed by monitoring the initial conversion rate of nitrobenzene along with azoxybenzene catalyzed by Rod-CeO₂. The corresponding rates were r₁=0.185 mmol g⁻¹ h⁻¹ and r₂=0.075 mmol g⁻¹ h⁻¹ respectively (Scheme S1 in Supporting information). Taking the previous mechanistic observations of related reaction systems and our experimental analysis into consideration, a surface-redox reaction pathway on different active sites of ceria catalysts for azoxybenzene and by-product azobenzene production is proposed. The conversion of nitrobenzene to azoxybenzene rely upon the presence of surface Ce³⁺ sites. Then azoxybenzene may undergo a further deoxygenation occurring on the lattice defects to produce the by-product azobenzene. This route is well supported by the above preliminary mechanistic results of Rod-CeO₂ catalyst. Compared with other ceria catalysts, Rod-CeO₂ exhibits the highest Ce³⁺ proportion but relatively lower surface area, which is related to the much higher conversion rate of nitrobenzene than that of azoxybenzene for Rod-CeO₂ and causes the inhabitation of azobenzene production.

In conclusion, the physico-chemical properties of the series of CeO₂ samples with different morphologies and their catalytic performance were explored in this work. The significant enhancement of azoxybenzene yield with the increasing Ce³⁺ proportion indicates that the surface Ce³⁺ proportion is the key ingredient for azoxybenzene synthesis. With the highest surface Ce³⁺ proportion, the Rod-CeO₂ exhibited completely conversion of nitrobenzene and 89.8% azoxybenzene selectivity in 7 h at 150 ℃ under 1 MPa CO. The preliminary mechanistic analysis clarified the high selectivity of azoxybenzene of Rod-CeO₂ by indicating the inhabitation of azoxybenzene to by-product azobenzene. Therefore, synthesizing surface-Ce³⁺-richer CeO₂ catalysts is proposing for the green synthesis of azoxybenzene and further research.

Declaration of competing interest

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

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Nos. 51836006, U1609212), and Key Research and Development Project of Shandong Province (No. 2019JZZY010403).

Appendix A. Supplementary data

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.cclet.2020.05.023.