The influence of alumina phases on the performance of Pd/Al2O3 catalyst in selective hydrogenation of benzonitrile to benzylamine

Various phase alumina supports were prepared by heating commercial pseudoboehmite at 400 °C and 1200 °C to obtain gamma (γ) and alpha (α) Al2O3, and by calcinations of synthetic bayerite at 400 °C and 1000 °C to obtain Eta (η) and theta (θ) Al2O3. These supports were then impregnated and calcined to prepare 0.5 wt% Pd catalysts. The prepared catalysts were characterized by XRD, BET, NH3–TPD, CO–TPD, H2–TPR, etc., and their catalytic performances were evaluated for selective hydrogenation of benzonitrile. In the order of Pd/γ-Al2O3, Pd/η-Al2O3, Pd/θ-Al2O3 and Pd/α-Al2O3, the surface areas decreased from 265 to 3.8 m2/g, the number of acidic sites was decreased, especially for strong acid sites, and meanwhile, the Pd size increased from 3.0 to 36.9 nm. The benzonitrile conversion showed limited relationship with the nature of catalysts and assumed to rely on the H2 transmission rate. The dibenzylamine formation was considered to be a result of competitive reactions and performed as a function of the metal dispersion. Among the four Pd/Al2O3 catalysts, the Pd/γ-Al2O3 and Pd/η-Al2O3 both exhibited excellent stability and catalytic performance with a high benzylamine yield of ca. 90%.