Nitrogen-Doped Activated Carbon as Metal-Free Catalysts Having Various Functions

Nitrogen-doped carbon materials have been gaining increasing interest as metal-free catalysts. In this article, the authors have briefly introduced their recent studies on the utilization of nitrogen-doped activated carbon (N-AC) for several organic synthesis reactions, which include base catalyzed reactions of Knoevenagel condensation and transesterification, aerobic oxidation of xanthene and alcohols, and transfer hydrogenation of nitrobenzene, 3-nitrostyrene, styrene, and phenylacetylene with hydrazine. Doped-nitrogen species existed on the AC surface in different structures. For example, pyridine-type nitrogen species appear to be involved in the active sites for Knoevenagel condensation and for the oxidation of xanthene, while graphite-type nitrogen species appear to be involved for the oxidation of alcohols. Being different from these reactions, both surface nitrogen and oxygen species are involved in the active sites for the hydrogenation of nitrobenzene. N-AC was practically inactive for the transfer hydrogenation of vinyl and ethynyl groups, but it can catalyze those hydrogenation reactions assisted by co-existing nitrobenzene. Comparison of N-AC with conventional catalysts shows that N-AC can alternate with conventional solid base catalysts and supported metal catalysts for the Knoevenagel condensation and oxidation reactions.