β-MnO2 nanoparticles as heterogenous catalysts for aerobic oxidative transformation of alcohols to carbonyl compounds, nitriles, and amides

Abstract

β-MnO₂ nanoparticles with high specific surface areas were successfully synthesized by the crystallization of a layered manganese oxide precursor prepared using not only NaMnO₄, but also inexpensive and easily available KMnO₄. These β-MnO₂ nanoparticles could function as an effective and reusable solid catalyst for the aerobic oxidation of various aromatic and heteroaromatic alcohols to the corresponding carbonyl compounds with molecular oxygen as the sole oxidant. β-MnO₂ exhibited higher catalytic activity than other catalysts, including manganese-based simple and complex oxides under mild reaction conditions. In addition, the present oxidation system could be applied to the one-pot tandem oxidative transformation of alcohols to the corresponding nitriles and amides in the presence of ammonia, without the need for any additives such as strong bases and nitroxyl radicals. β-MnO₂ nanoparticles were found to be more effective catalysts for the selective synthesis of nitriles from alcohols than the α-MnO₂ based OMS-2 catalyst, likely due to the high oxidation activity and low nitrile hydration activity of β-MnO₂. Mechanistic studies including the poisoning effects for nitrile hydration showed that the differences in acid sites and crystal structures between β-MnO₂ and OMS-2 likely affect the activation of the nitriles and water, respectively, which leads to the high selectivity of β-MnO₂ for nitrile synthesis.