Efficient Cu–Ni/W20O58 Catalysts for Hydrogenation of Nitriles to Secondary Amines

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 Cu_4.4–Ni_0.6/W₂₀O₅₈ catalyst showed a 92.3% yield with initial TOF_s 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 Cu_4.4/W₂₀O₅₈ and Ni_0.6/W₂₀O₅₈ catalysts. Moreover, the Cu_4.4–Ni_0.6/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 Cu_4.4–Ni_0.6/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 Cu_4.4–Ni_0.6/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 Cu_4.4–Ni_0.6/W₂₀O₅₈ catalyst.