Application of complex heteropolytungstates in limonene epoxidation by H 2O 2 in biphasic medium

Abstract

The synthesis of potassium salts of Cu heteropolytungstate was carried out by reaction in aqueous solution of Na 8H[ΔPW 9O 34]·19H 2O lacunary precursor with Cu nitrate in KCl excess. The product is a mixture of two isomers related to Keggin structure, having a composition K 5.5Na 1.5[PW 10O 38Cu 2(H 2O) 2]·13H 2O, which are the main phases, while the tetra-nuclear copper cluster with formula: [K 10(PW 9O 34) 2Cu 4(H 2O) 2]·20H 2O 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 H 2O 2) 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 ([K 10(PW 9O 34) 2Co 4(H 2O) 2]·20H 2O) 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.