This paper examines the reactivity of catalysts based on niobium oxide dispersed in silica, for the oxidation of 2-methyl-1-naphthol to 2-methyl-1,4-naphthoquinone(menadione) with aqueous hydrogen peroxide. The catalytic performance is affected by the hydroperoxide concentration, and by the mode of oxidant addition. The reaction scheme is rather complex, including, along with the desired reaction, also parallel and consecutive side processes. The catalyzed oxidation reaction to yield menadione is always accompanied by radical reactions leading to coupling products. The use of diluted hydrogen peroxide solutions (concentration 3.5 wt.%) favors the formation of these heavy compounds. On the other hand, in the presence of concentrated hydrogen peroxide solution (35 wt.%), the oxidation of the substrate to menadione occurs even without catalyst. High selectivity to menadione (i.e., 93%) is achieved only at the very beginning of the reaction when the substrate is added to the catalyst-containing hydrogen peroxide solution, the oxidant concentration being 35%. Under the latter conditions, the low concentration of substrate disfavors the undesired coupling reactions. (C) 2009 Elsevier B.V. All rights reserved.