Highly Efficient Oxidative Dehydrogenation Aromatization of 1,2,3,4-Tetrahydroquinoline by Cu2-MnOx Catalyst
Acta Phys. -Chim. Sin.
A novel template-free oxalate route was applied to synthesize a series of MnOx catalysts with different Cu content (MnOx, Cu1-MnOx, Cu2-MnOx, Cu3-MnOx, Cu4-MnOx, Cu2-450, and Cu2-550), which were then used in 1,2,3,4-tetrahydroquinoline (THQL) oxidative dehydrogenation aromatization. To obtain insight into the structure-activity relationships of the catalysts, the samples were characterized by thermogravimetry and heat flow analysis, X-ray diffraction (XRD), N2 physical adsorption-desorption, scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), H2 temperature programmed reduction (H2-TPR), and atomic absorption spectroscopy (AAS). The results showed that Cu2-MnOx possesses the following characteristics:amorphous nature, high specific surface area, increased mesoporous average pore diameter, lower reduction temperature, highest Mn3+ and adsorbed oxygen content, and highest Mn3+/Mn4+ ratio among the seven manganese oxide catalysts. Cu2-MnOx for the oxidative dehydrogenation aromatization of THQL showed conversion (99.1%) and selectivity (97.2%) for quinoline under mild reaction conditions, with cheap air as oxidant and no alkali additive. Cu2-MnOx was reusable and achieved 95.8% conversion even after five reuse tests. Selectivity decreased slightly with the increase in reuse time, which could be attributed to the leaching of the Cu element. Comparison of structure-activity relationships showed increased catalytic activity when Mn3+ and adsorbed oxygen content were highest among these amorphous manganese oxides. Mn4+ content was related to the formation of quinoline N-oxide by over oxidation. Despite their high Mn3+ content and Mn3+/Mn4+ ratio, Cu2-450 and Cu2-550 had reduced surface area, adsorbed oxygen content, and lattice oxygen mobility, which resulted in poor catalytic performance. Although Cu3-MnOx had the largest BET surface area, highest lattice oxygen mobility, and similar Mn3+ and adsorbed oxygen content as Cu2-MnOx, the smaller average pore diameter of Cu3-MnOx perhaps caused its conversion and selectivity to be similar to Cu2-MnOx. The amorphous nature, Mn3+ and adsorbed oxygen content, Mn3+/Mn4+ ratio, lattice oxygen mobility, and synergistic effect between CuO and MnOx were found to play key roles in catalytic performance. The absence of precious metals, the simple catalyst preparation process, the cheap air as the sole oxidant, no ligand and alkali, the mild reaction conditions, along with catalyst reusability and easy isolation of the aromatized products made our catalytic protocol both green and environmentally benign.
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