The hydrogenation of nitriles is an environmentally friendly and atom-economical route to prepare high-value amines; however, it is still a challenge to control selectivity because of the occurrence of hydrogenation, self-coupling, and hydrogenolysis reactions during the hydrogenation process. It is highly desirable to develop efficient non-noble-metal catalysts for the hydrogenation of nitriles to secondary amines, although several noble metal catalysts have been reported. Herein, we successfully prepared a Cu–Ni/W20O58 catalyst and found that the Cu4.4–Ni0.6/W20O58 catalyst showed a 92.3% yield with initial TOFs of 270.0 h–1 at 373 K and 3 MPa H2 for the hydrogenation of benzonitrile (BN) to dibenzylamine (DBA), which was much superior to that of the monometallic Cu4.4/W20O58 and Ni0.6/W20O58 catalysts. Moreover, the Cu4.4–Ni0.6/W20O58 catalyst could be reused at least 4 times and was effective for the hydrogenation of various nitriles with superior selectivity to secondary amines. Furthermore, the reaction mechanism of BN hydrogenation for the Cu–Ni alloy of Cu4.4–Ni0.6/W20O58 was proposed on the basis of spectroscopic studies such as XRD, TPR, TEM, XAFS, and XPS, kinetic studies such as the effect of BN concentration and H2 pressure, the isotopic effect of hydrogen, and the effect of the reaction temperature and DFT calculations. The rate-determining step was the hydrogenation of benzylidenamine (BI) to the half-hydrogenated intermediate by one H atom over the Cu4.4–Ni0.6/W20O58 catalyst. The Ni–W20O58 interface was responsible for the adsorption and activation of BN, and the electron-rich Cu acted as the site for H2 dissociation; the synergistic effect of Cu and Ni led to the superior catalytic performance of the Cu4.4–Ni0.6/W20O58 catalyst.
The development of new heterogeneous non-noble catalytic systems for the selective hydrogenation of nitriles into primary amines is a challenging task. In this study, a mesoporous Al2O3 supported Ni catalyst (denoted as Ni/Al2O3-600, where 600 represents the reduction temperature) demonstrated a high catalytic activity for the hydrogenation of nitriles under mild conditions (60–80 °C and 2.5 bar H2) with ammonia as the additive. This catalytic system has a wide substrate range and the Ni/Al2O3 catalyst demonstrated a good tolerance to other functional groups, which was possibly due to its high catalytic activity under mild conditions. A plausible reaction pathway was proposed for the hydrogenation of nitriles into primary amines, and it was found that ammonia played a great role in the enhancement of the selectivity of primary amines by the inhibition of the side reaction to generate secondary amines. In addition, the Ni/Al2O3-600 catalyst could be reused five times without activity loss through convenient magnetic recovery.
A novel explosion-assistant strategy was developed to fabricate graphene oxide supported nickel and palladium three-component catalyst Pd/Ni/NiO/GO for hydrogenation of nitriles to secondary amines. The explosion during the synthesis of catalyst was essential to the well-dispersion of palladium and the decomposition of metal salt precursors. Palladium component was well anchored on nickel oxide and graphene oxide sheets, which regulated the catalytic performance of palladium. This special structure facilitated the spillover of active hydrogen species generated on palladium component to nickel (oxide) and graphene oxide sheets, enhancing the catalytic activity of the bimetallic catalyst. This work developed an effective strategy to fabricate multi-component metal catalyst with low-cost transition metal as main composition for selective hydrogenation of nitriles to secondary amines.
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