Highly efficient continuous-flow oxidative coupling of amines using promising nanoscale CeO2-M/SiO2 (M = MoO3 and WO3) solid acid catalysts

The development of promising solid acid catalysts alternative to hazardous liquid acids is essential towards a sustainable chemical industry. This work reports the synthesis of nanostructured CeO 2 -MoO 3 /SiO 2 and CeO 2 -WO 3 /SiO 2 solid acids, along with CeO 2 -MoO 3 , CeO 2 -WO 3 and CeO 2 for continuous-flow oxidative coupling of benzylamine using O 2 as a green oxidant. A systematic physicochemical characterization has been undertaken using XRD, Raman, N 2 adsorption-desorption, TEM, NH 3 -TPD, and XPS techniques. It was found that the dispersion of CeO 2 -MoO 3 and CeO 2 -WO 3 species on the SiO 2 support leads to remarkable structural and acidic properties, due to the synergetic effect of the respective components. TEM analysis reveals the presence of highly dispersed WO 3 (0.8-1.2 nm) and MoO 3 (0.8-1 nm) nanoparticles in the synthesized catalysts. Among the various catalysts developed, the CeO 2 -MoO 3 /SiO 2 sample exhibited higher BET surface area (248 m 2 g -1 ), abundant oxygen vacancy defects, and large amounts of strong acidic sites. Owing to improved properties, the CeO 2 -MoO 3 /SiO 2 solid-acid showed a superior catalytic performance in the continuous-flow oxidative coupling of benzylamine: the obtained benzylamine conversions for 1 h are ∼11.8, 55, 70, 76, and 96%, respectively, for CeO 2 , CeO 2 -WO 3 , CeO 2 -WO 3 /SiO 2 , CeO 2 -MoO 3 , and CeO 2 -MoO 3 /SiO 2 catalysts. Importantly, the CeO 2 -MoO 3 /SiO 2 solid acid exhibited a remarkable steady performance in terms of benzylamine conversion (∼88-96%) and selectivity of N-benzylbenzaldimine product (∼96-97.8%) up to 6 h. The outstanding catalytic performance of CeO 2 -MoO 3 /SiO 2 solid acid coupled with the application of continuous-flow synthesis, economical benefits of the respective oxides, and eco-friendly oxidant is expected to bring new opportunities in the design of industrially-favourable chemical processes.