Rb-promoted Fe/CeO2 nanocatalyst for aniline conversion into azoxybenzene, DFT calculations and mechanism

Abstract

Selectivity control of aniline oxidation and low reactant conversion in traditional synthesis methods are great challenges, and it is desirable to develop a green, low-cost and highly efficient catalytic route toward value-added products. Herein, an Rb-promoted Fe/CeO₂ nanocatalyst was prepared to understand the effects of Rb-promoter on the catalytic performance for the selective oxidation of aniline to azoxybenzene using H₂O₂ as an oxidant. The 0.1 M Rb-4% Fe/CeO₂ (Rb–Fe/CeO₂) catalyst showed a high aniline conversion of 100% with 91% selectivity of azoxybenzene. This is because the existence of Rb contributes to the electron transportation property, decreases activation energy and leads to lattice distortion of Fe/CeO₂ and further formation of oxygen vacancies and Ce³⁺, which contributes to improving the activity of Fe/CeO₂ nanocatalysts for aniline conversion reaction. The Rb used to modify Fe/CeO₂ nanocatalysts can not only passivate the strong Brønsted acid sites and stability of Fe/CeO₂ but also enhance the Fe dispersion and induce an electron-rich chemical environment for the supported Fe species and promote the activation of the substrate. All these effects lead to the desirable catalytic performance. The increased basic strength of the cation-promoted catalyst improves the electron density of the active Fe species, resulting in a higher yield of the desired aromatic azo compounds. This compensates for electronic deficiencies in the Fe, enhancing its catalytic activity without interference. Experiments were conducted as a function of catalyst loading (20–100 mg), time (2–24 h), temperature (25–100 °C), types of solvent and solvent amount (0.5–2 ml) in 50 ml round bottom flask with reflux condenser. Our work proposes a facile approach to develop and promote non-noble metal catalysts for the effective conversion of aniline into azoxybenzene under mild reaction conditions.