Oxidation of 2-methyl-1-naphthol with H2O2 over mesoporous Ti-MMM-2 catalyst

The catalytic oxidation of 2-methyl-1-naphthol (MNL) to 2-methyl-1,4-naphthoquinone (MNQ; menadione; vitamin K3) using aqueous H2O2 as an oxidant and hydrothermally stable mesoporous mesophase titanium silicates Ti-MMM-2 as catalysts has been studied. The effects of mesopore diameter, temperature, solvent nature, reagent concentrations, and catalyst amount on the yield of MNQ were evaluated. Crucial factors affecting the MNQ yield include MNL concentration, H2O2/MNL molar ratio, reaction temperature, and the mode of MNL addition to the reaction mixture. Selectivity dramatically decreases with increasing MNL concentration and decreasing temperature. Stepwise addition of the substrate to the reaction mixture improves selectivity. Oxidation in the presence of 18O2 provided image of unlabeled MNQ, indicating H2O2 as an oxygen source. The Ti-MMM-2 catalyst allows one to obtain MNQ with selectivity as high as 78% at complete substrate conversion. The catalyst can be used repeatedly without loss of either activity or selectivity. The proposed method is convenient and environmentally benign.