Selective oxidation of limonene over sodium salt of cobalt containing sandwich-type polyoxotungstate [WCo3(H2O)2{W9CoO34}2]10−

Abstract

A sandwich-type polyoxotungstate, Na10[Co5W19O70H4]·44H2O, was synthesized as the sodium salt from an aqueous solution containing sodium tungstate and cobalt nitrate in acidic medium by known methods. This was characterized by atomic absorption spectroscopy, IR, UV-Vis spectroscopy and thermogravimetric analysis (TG) and single crystal X-ray analysis and used as a heterogeneous catalyst for the oxidation of limonene using air or aqueous hydrogen peroxide as oxidants to form the epoxide, carveol and carvone. With either oxidant, one of the olefinic bonds of limonene is selectively oxidized to give the epoxide as the primary product. With H2O2, the efficiency was poor leading to lower conversion of limonene. The polyoxotungstate is able to activate molecular oxygen at 80–120 ◦C, under 1.5MPa pressure and oxidize limonene selectively.