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

• Nanostructured Au/CeO2 and Au/Ce1-xZrxO2 catalysts were synthesized.
• Oxidative coupling of benzylamines into N-benzylbenzaldimines is investigated.
• Au/Ce1-xZrxO2 catalyst exhibited excellent catalytic activity.
• Superior selectivity (∼99.6–99.9%) for N-benzylbenzaldimines was observed.

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 CeO2 and CeO2-ZrO2 supports for solvent- and base-free oxidative coupling of benzylamines into N-benzylbenzaldimines using O2 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 CeO2 and CeO2-ZrO2 supports with an average diameter of ∼10 and 7 nm, respectively. HRTEM images also indicated that Au/CeO2-ZrO2 catalyst has smaller Au nanoparticles (∼2.1 nm) compared with that of Au/CeO2 catalyst (∼3.7 nm). Raman and XPS studies showed that the addition of ZrO2 to CeO2 leads to abundant oxygen vacancies and higher concentration of Ce3+, respectively. The Au/CeO2-ZrO2 catalyst exhibited a higher efficiency in benzylamine conversion (∼95%) followed by Au/CeO2 (∼78%), CeO2-ZrO2 (∼51%), and CeO2 (∼39%). The Au/CeO2-ZrO2 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 CeO2-ZrO2 support are found to be key factors for high performance of Au/CeO2-ZrO2 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.

Nano-Au dispersed on nanocrystalline CeO2-ZrO2 support exhibits a higher catalytic efficiency in the solvent-free oxidative coupling of benzylamine, which is attributed to catalytically favorable properties of nano-Au and the CeO2-ZrO2 support.Figure optionsDownload high-quality image (141 K)Download as PowerPoint slide