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 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.
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