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

Abstract

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 CeO₂–MoO₃/SiO₂ and CeO₂–WO₃/SiO₂ solid acids, along with CeO₂–MoO₃, CeO₂–WO₃ and CeO₂ for continuous-flow oxidative coupling of benzylamine using O₂ as a green oxidant. A systematic physicochemical characterization has been undertaken using XRD, Raman, N₂ adsorption–desorption, TEM, NH₃-TPD, and XPS techniques. It was found that the dispersion of CeO₂–MoO₃ and CeO₂–WO₃ species on the SiO₂ 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 WO₃ (0.8–1.2 nm) and MoO₃ (0.8–1 nm) nanoparticles in the synthesized catalysts. Among the various catalysts developed, the CeO₂–MoO₃/SiO₂ sample exhibited higher BET surface area (248 m² g⁻¹), abundant oxygen vacancy defects, and large amounts of strong acidic sites. Owing to improved properties, the CeO₂–MoO₃/SiO₂ 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 CeO₂, CeO₂–WO₃, CeO₂–WO₃/SiO₂, CeO₂–MoO₃, and CeO₂–MoO₃/SiO₂ catalysts. Importantly, the CeO₂–MoO₃/SiO₂ 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 CeO₂–MoO₃/SiO₂ 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.