Liquid phase oxidation of limonene catalyzed by palladium supported on hydrotalcites

Palladium supported on copper-magnesium-aluminium hydrotalcites (HTs) were prepared by using coprecipitation and impregnation techniques. The different palladium/hydrotalcites solid catalysts were tested in the liquid phase oxidation of pure limonene using molecular oxygen as the sole oxidant agent. Both palladium and copper were found to influence the conversion versus reaction time profile of limonene oxidation. A chain reaction mechanism is proposed to explain the experimental results. Palladium participates in the formation of allylic radicals through activation of carbon-hydrogen bonds of limonene and in the formation of adducts by cleavage of the carbon-carbon double bonds. Copper catalyzes mainly the decomposition of hydroperoxides leading to radical intermediates. Different oxygen containing derivates from limonene were formed (1,2- and 8,9-epoxylimonene, cis and trans carveol and carvone) according to the chain reaction mechanism. Higher reaction rates and final conversion of limonene over 70% were obtained with a suitable content of copper in the hydrotalcite. (C) 2003 Elsevier B.V. All rights reserved.