Limonene oxidation by molecular oxygen under solvent-free conditions: the influence of peroxides and catalysts on the reaction rate

Liquid-phase oxidation of limonene was performed with molecular oxygen as the sole oxidizing agent under non-solvent conditions. The oxidation took place under atmospheric pressure and temperatures between 70 and 90 A degrees C in the presence of three different nickel-aluminium hydrotalcites. The final limonene conversion at 80 A degrees C and 6 h of reaction time was comprised between 40 and 50 %. Several oxygenated limonene derivatives such as endo- and exo-epoxides, carveol and carvone represent 45-60 % of the whole amount of products. In the absence of a catalyst, a higher initial content of peroxides in limonene lead to a higher reaction rate with no significant changes in product selectivities. Thermal decomposition of limonene peroxides took place as the initiation path of an autooxidation mechanism leading to the different products. The catalyst played an important role in the initial paths involving activation of both reactants as well as in the decomposition of limonene peroxide to form radicals.