Application of complex heteropolytungstates in limonene epoxidation by H2O2 in biphasic medium

The synthesis of potassium salts of Cu heteropolytungstate was carried out by reaction in aqueous solution of Na8H[ΔPW9O34]·19H2O lacunary precursor with Cu nitrate in KCl excess. The product is a mixture of two isomers related to Keggin structure, having a composition K5.5Na1.5[PW10O38Cu2(H2O)2]·13H2O, which are the main phases, while the tetra-nuclear copper cluster with formula: [K10(PW9O34)2Cu4(H2O)2]·20H2O was the minority phase. The characterization was carried out by means of X-ray powder diffraction analysis (XRD) and vibrational FT-IR and Raman spectroscopy. The Cu-phase mixture was used as catalyst for the epoxidation of limonene in biphasic (water-organic) reaction media using hydrogen peroxide (35% aqueous H2O2) as the oxygen donor. It was able to oxidize selectively limonene towards the epoxide at very high turnover numbers. In addition, similar studies were performed with both the structural related ([K10(PW9O34)2Co4(H2O)2]·20H2O) and the lacunary precursor for comparative purposes. Some comments about the stability of copper and cobalt tetra-nuclear phases were made on the basis of topological and electronic considerations and the distortion of the metal environments was related to the catalytic behavior.