New insight on the mechanism of the catalytic hydrogenation of nitrobenzene to 4-aminophenol in CH3CN-H2O-CF3COOH as a reusable solvent system. Hydrogenation of nitrobenzene catalyzed by precious metals supported on carbon

The preparation of 4-aminophenol via hydrogenation of nitrobenzene in a single liquid phase has been carried in the presence of different precious metal catalysts. The liquid phase is composed of CH3CN-H2O-CF3COOH, which can be easily distilled at low temperature, thus avoiding work-up operation. The yield of 4-aminophenol is in the range 45% and in the presence of sulfolane as promoter reaches almost 50%. The best result has been obtained in the presence of Pt/C as hydrogenation catalyst. The role of the solvent is strictly related to the selectivity to 4-aminophenol, since CH3CN decreases the hydrogenation activity compared to other solvent, CF3COOH promotes the formation of the desired product both via Bamberger rearrangement in solution as well by a surface catalyzed reaction, while H2O is responsible for 4-aminophenol formation in both reactions. Even though, nitrosobenzene and phenylhydroxylamine have not been observed, their reactivity suggest a complex pattern of reactions occurring either on the catalyst surface or in the solution. Indeed, formation of 4-aminophenol may occur both in solution, via acid catalyzed Bamberger rearrangement and on the catalyst surface by the formation of a surface Pt-nitrenium complex, which undergoes surface nucleophilic attack by H2O. (C) 2014 Elsevier B.V. All rights reserved.