Accurate Understanding the Catalytic Role of MnO2 in the Oxidative-Coupling of 2-naphthols into 1,1′-bi-2-naphthols

It has been reported that β -MnO 2 has photocatalytic activity for the oxidative-coupling of 2-naphthols into 1,1′-bi-2-naphthols. Nevertheless, it is hard to exclude the possibility that the oxidative-coupling of 2-naphthols is initiated by β -MnO 2 catalysis in dark due to the insufficient investigations in the related reports. In the present work, the oxidative-coupling of 2-naphthols into 1,1′-bi-2-naphthols with different phases MnO 2 catalysis in dark and under visible-light irradiation were systematically investigated. The results revealed that the oxidative-coupling of 2-naphthols is jointly initiated by MnO 2 catalysis and O 2 -oxidation, not by MnO 2 photocatalysis. Among the α -MnO 2 , β -MnO 2 , γ -MnO 2 and δ -MnO 2 catalysis, β -MnO 2 catalysis has the optimal performance, its selectivity for the oxidative-coupling of 2-naphthols into 1,1′-bi-2-naphthols is close to 100%, and its catalytic capacity could be well retained after multiple using. Our findings provide comprehensive and accurate understanding the catalytic role of MnO 2 for the oxidative-coupling of 2-naphthols into 1,1′-bi-2-naphthols. Graphic In the present work, the oxidative-coupling of 2-naphthols into 1,1′-bi-2-naphthols was proved to be jointly initiated by MnO 2 catalysis and O 2 -oxidation, not by MnO 2 photocatalysis. β -MnO 2 has the optimal catalytic activity for the oxidative-coupling of 2-naphthols relative to α -MnO 2 , γ -MnO 2 and δ -MnO 2 .