Insight into the Role of Additives in Catalytic Synthesis of Cyclohexylamine from Nitrobenzene

Cyclohexylamine is a versatile intermediate in various chemical industries, which can be expediently synthesized via hydrogenation of aniline or nitrobenzene. However, such processes always suffer from side reactions. Many reports have found that specific additives can suppress the formation of side products, but the mechanism is still not clear. In this work, results suggest that it is the hydroxide ion of alkali metal hydroxides, rather than the cations, which plays a key role in suppressing the side reactions on supported Ru-based catalysts. With the assistance of LiOH, the selectivity toward cyclohexylamine increased from 85.4% to 100%. Side products, such as dicyclohexylamine, cyclohexanol and N-isopropyl cyclohexylamine, could no longer be detected. Theoretical calculations further disclosed that addition of alkali metal hydroxides inhibited the dissociation of enamine and decreased the adsorption energy of cyclohexylamine, which might be the reasons for a better selectivity. However, the addition of alkali metal hydroxides reduced the activity of nitrobenzene hydrogenation by unfolding the condensation reaction route. To recover or even further enhance the catalytic performance, a second metal component was introduced and the resultant RuNi/AC exhibited a significant improvement in activity compared with Ru/AC.