Selectivity in the Phase Transfer Catalytic Hydrogenation of Nitrobenzene by Transition VIII Metals

The phase transfer hydrogenation of nitrobenzene to p‐aminophenol has been investigated for the catalysts Pt, Pd, Rh and Ru. The influence of factors such as surfactant concentration, sulfuric acid concentration, hydrogen pressure and temperature on the reaction was determined. It was found that the rate‐determining step for the catalytic hydrogenation of nitrobenzene is the catalytic process by which the hydrogen atom is absorbed on the metal surface. A reaction model is suggested which accounts for the observed hydrogenation selectivity and explains how these factors affect product composition and yield. It is shown that Pt favors p‐aminophenol production, while Pd favors aniline production.