Integrated Experimental and Theoretical Study of Shape-ControlledCatalytic Oxidative Coupling of Aromatic Amines over CuO Nanostructures

摘要

We have synthesized CuO nanostructures with flake, dandelion-microsphere, and short-ribbon shapes using solution-phase methods and have evaluated their structure–performance relationship in the heterogeneous catalysis of liquid-phase oxidative coupling reactions. The formation of nanostructures and the morphological evolution were confirmed by transmission electron microscopy, scanning electron microscopy, X-ray diffraction analysis, X-ray photoelectron spectroscopy, Raman spectroscopy, energy-dispersive X-ray spectroscopy, elemental mapping analysis, and Fourier transform infrared spectroscopy. CuO nanostructures with different morphologies were tested for the catalytic oxidative coupling of aromatic amines to imines under solvent-free conditions. We found that the flake-shaped CuO nanostructures exhibited superior catalytic efficiency compared to that of the dandelion- and short-ribbon-shaped CuO nanostructures. We also performed extensive density functional theory (DFT) calculations to gain atomic-level insight into the intriguing reactivity trends observed for the different CuO nanostructures. OurDFT calculations provided for the first time a detailed and comprehensiveview of the oxidative coupling reaction of benzylamine over CuO, whichyields N-benzylidene-1-phenylmethanamine as the majorproduct. CuO(111) is identified as the reactive surface; the specificarrangement of coordinatively unsaturated Cu and O sites on the moststable CuO(111) surface allows N–H and C–H bond-activationreactions to proceed with low-energy barriers. The high catalyticactivity of the flake-shaped CuO nanostructure can be attributed tothe greatest exposure of the active CuO(111) facets. Our finding shedslight on the prospective utility of inexpensive CuO nanostructuredcatalysts with different morphologies in performing solvent-free oxidativecoupling of aromatic amines to obtain biologically and pharmaceuticallyimportant imine derivatives with high selectivity.