Solvent-free liquid-phase autoxidation of monoterpenes catalyzed by sol-gel CO/SiO2

Oxidation of limonene, 3-carene, alpha-pinene and beta-pinene with dioxygen promoted by heterogeneous sol-gel Co/SiO2 catalysts has been performed under solvent-free conditions. Limonene and alpha-pinene give both epoxidation and allylic oxidation products, with chemoselectivities for the latter being 50-60% at a 40% substrate conversion. On the other hand, oxidation of beta-pinene leads essentially to allylic products, i.e., highly valuable pinocarveol, pinocarvone, myrtenol and myrtenal. Good combined selectivities of up to 88% at ca. 40% conversion have been achieved. The activity of the Co/SiO2 catalysts is strongly influenced by the temperature of their thermal treatment. No cobalt leaching occurs under the reaction conditions and the catalysts can be recovered and reused several times. (C) 2004 Elsevier B.V. All rights reserved.