Highly selective direct amination of primary alcohols over a Pd/K-OMS-2 catalyst

A new Pd-substituted octahedral molecular sieve (Pd/K-OMS-2) catalyst has been prepared for the direct amination of alcohols with primary amines operating under the borrowing hydrogen mechanism. The catalyst offered full conversion and high selectivity toward N-benzylaniline in the model alkylation reaction of aniline with benzyl alcohol at mild temperature (160 °C) for 3 h with neither production of the tertiary amine nor toluene. Pd/K-OMS-2 performed as a tandem tri-functional catalyst, first oxidizing benzyl alcohol to benzaldehyde, behaving as a Lewis acid for imine formation, and finally reducing the imine to the secondary amine. The catalyst was characterized in depth using BET, XRD, H2-TPR, XPS, FTIR, TEM, TGA/DTG, and ICP-AES / EDX to elucidate the nature of the active sites. The unexpectedly high performance of the Pd/K-OMS-2 catalyst can be ascribed, at least partially, to the in situ generation of a very active, selective and partially recyclable Pd-substituted/supported hausmannite phase (i.e., Pd/Mn3O4) in the early stage of the reaction with a high density of surface oxygen moieties. We argue about a possible role of a Pd(IV)/Pd(II) redox pump for exchanging hydrogen during the amination reaction.