Hollow CuO–CeO₂ Nanospheres for an Effectively Catalytic Annulation/A³-Coupling Reaction Sequence

Abstract

A heterogeneous catalyst CuO–CeO2 with a hollow structure was successfully synthesized by assembling CuO on the outer surface of CeO2 hollow spheres through a layer-by-layer deposition strategy. Further reduction under a H2 atmosphere was carried out to modify its surface state, resulting in sample r-CuO–CeO2. The catalysts were characterized by X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform-infrared spectroscopy (FT-IR). SEM and TEM showed that the synthesized catalysts had a hollow sphere structure, which can expose more active sites and promote mass transfer. The results of XPS revealed that the content of Cu0&Cu+ increased from 13.6 to 19.0% after H2 reduction, which was conducive to the activation of terminal alkyne to promote the annulation/A3-coupling reaction. The conversion of this cascade reaction with the r-CuO–CeO2 catalyst can reach more than 95%, while the value was only 55% by using CuO–CeO2 as a catalyst. Moreover, the r-CuO–CeO2 catalyst showed a good recycle property and group compatibility in the general applicability of the cascade annulation/A3-coupling reaction, providing a series of propargylamine derivatives in good chemoselectivity.

CuS Morphology Control for Enhanced Catalytic Performance in A3 Coupling Reaction

“…The characteristic peaks at 568.3, 569.0, 570.2 eV in the Auger spectrum of Cu LMM proved the coexistence with Cu + , Cu 2+ and Cu 0 in the sample (Figure 4c). [24] According to previous studies, low valence copper is beneficial to the catalytic coupling reaction, and catalysts with high Cu + /Cu 0 content tend to have better catalytic activity [12] . In Figure 4d, the high‐resolution spectrum of S 2p is divided into four peaks with the corresponding fitting analysis results being presented in Table 1.…”

“…The three different morphologies of CuS showed different properties, especially the different content of Cu + /Cu 0 and S n 2− on the surface, which had a powerful effect on the catalytic activity. Previous studies have shown that low valence copper species Cu + /Cu 0 are conducive to the activation of C−H and can promote the reaction [12,27] . S n 2− provides Lewis acid sites, enhances the adsorption of reaction molecules, and is helpful to the cleavage of C−H bonds [28] .…”

“…[24] According to previous studies, low valence copper is beneficial to the catalytic coupling reaction, and catalysts with high Cu + /Cu 0 content tend to have better catalytic activity. [12] In Figure 4d, the highresolution spectrum of S 2p is divided into four peaks with the corresponding fitting analysis results being presented in Table 1. The peaks with binding energies of 162 and 163 eV can be attributed to lattice S 2À , and the peak of SÀ O bond appears at about 169 eV.…”

“…Previous studies have shown that low valence copper species Cu + /Cu 0 are conducive to the activation of CÀ H and can promote the reaction. [12,27] S n 2À provides Lewis acid sites, enhances the adsorption of reaction molecules, and is helpful to the cleavage of CÀ H bonds. [28] CuS NP has the highest Cu + /Cu 0 (58.9 %) and S n 2À (16.7 %), and shows the best catalytic performance.…”

“…[10] Chen et al synthesized CuS nanocrystals by a easy solvothermal process using ethylene glycol for the electrocatalytic reduction of carbon dioxide to formic acid and the catalyst displayed high Faraday efficiency (52 %). [11] Although CuS as catalyst in A 3 coupling reaction has not been reported, our previous work using CuO as active catalysts [12] for this reaction has provide the practicability of CuS, as both CuS and CuO have exposed Cu + /2 + sites. In addition, there are obvious differences in catalysts with different morphologies, in terms of surface energy, arrangement of atoms in crystals and concentration of surface defect sites, which have an impact on the adsorption and activation of reactants on the catalyst and thus influencing the catalytic activity.…”