Liquid-Phase Amination of Phenol to Aniline over the Pd/MgO Catalyst without External Hydrogen Addition

The amination of phenol to aniline is a sustainable method to replace the traditional synthesis of primary aniline by nitrobenzene-hydrogenation and halobenzene-amination. Under a hydrogen atmosphere, the supported Pd catalyst exhibits significant activity in phenol amination with various amines, but the selectivity to primary aniline is poor. Here, we showed that the phenol amination to aniline can be realized using a small amount of cyclohexanone as an inducer without the need for external hydrogen addition over the Pd/MgO catalyst. Under the condition of the cyclohexanone inducer, the selectivity of primary aniline can reach twice that of phenol amination under a hydrogen atmosphere, and the phenol conversion did not significantly decrease. Results indicated that the use of cyclohexanone as the inducer led to a system internally circulating the stoichiometric hydrogen species generated in situ, where the cyclohexanone readily reacted with ammonia to form a cyclohexanimine intermediate, which subsequently dehydrogenated to generate an aniline product and hydrogen species on the catalyst. The hydrogenation of phenol to cyclohexanone then occurred, but the reaction was regulated by the stoichiometric hydrogen species generated in situ in the system. Moreover, the produced aniline could readily desorb from Pd/MgO because of the limited acid density on the catalyst surface. All these phenomena benefited the reduction in the possibility of condensation of aniline with cyclohexanone to form a secondary amine and thus enhanced the aniline selectivity.