Promising nanostructured gold/metal oxide catalysts for oxidative coupling of benzylamines under eco-friendly conditions

Abstract Designing Au nanocatalysts supported on nanosized metal oxides has drawn much attention due to remarkable nanoscale influenced metal-support interactions and their favorable role in heterogeneous catalysis. This work reports development of Au nanocatalysts dispersed on nanosized CeO 2 and CeO 2 -ZrO 2 supports for solvent- and base-free oxidative coupling of benzylamines into N -benzylbenzaldimines using O 2 as a green oxidant. The physicochemical characterization of nanocatalysts has been undertaken using HRTEM, UV–vis DRS, XRD, Raman, BET, TG-DTA, AAS, and XPS techniques. HRTEM images reveal the formation of nanosized CeO 2 and CeO 2 -ZrO 2 supports with an average diameter of ∼10 and 7 nm, respectively. HRTEM images also indicated that Au/CeO 2 -ZrO 2 catalyst has smaller Au nanoparticles (∼2.1 nm) compared with that of Au/CeO 2 catalyst (∼3.7 nm). Raman and XPS studies showed that the addition of ZrO 2 to CeO 2 leads to abundant oxygen vacancies and higher concentration of Ce 3+ , respectively. The Au/CeO 2 -ZrO 2 catalyst exhibited a higher efficiency in benzylamine conversion (∼95%) followed by Au/CeO 2 (∼78%), CeO 2 -ZrO 2 (∼51%), and CeO 2 (∼39%). The Au/CeO 2 -ZrO 2 catalyst was also found to effective for oxidative coupling of various benzylamines, and moderate to good product yields were obtained. The presence of smaller Au particles (2.1 nm) and improved surface-defect properties of nanoscale CeO 2 -ZrO 2 support are found to be key factors for high performance of Au/CeO 2 -ZrO 2 catalyst. Additionally, the reaction temperature is one of the important factors for the performance of catalysts. Remarkably, ∼99.6–99.9% selectivity for N -benzylbenzaldimines was found in the amine oxidation, which highlights the significance of present work in the selective oxidation catalysis.