Eco-friendly solvents and amphiphilic catalytic polyoxometalate nanoparticles: a winning combination for olefin epoxidation

Abstract

Eighteen eco-friendly solvents were examined to carry out the epoxidation of olefins with the amphiphilic catalytic dodecyltrimethylammonium polyoxometalate nanoparticles [C₁₂]₃[PW₁₂O₄₀] in comparison with [H]₃[PW₁₂O₄₀] and [Na]₃[PW₁₂O₄₀]. Surprisingly, the screening of solvents with cyclooctene has revealed that the [C₁₂]₃[PW₁₂O₄₀] catalyst is much more active with initial turn-over frequencies, TOF₀, increasing up to a factor of 10. Moreover, the reaction occurs at competitive rates in four relevant solvents, i.e. cyclopentyl methyl ether, 2-methyl tetrahydrofuran, methyl acetate and glycerol triacetate, for which TOF₀ values are higher than 260 h⁻¹. The recyclability of the systems is demonstrated and the scope of substrates has been successfully extended to cyclohexene, 1-octene, limonene, 3-carene, α-pinene, β-pinene and neryl acetate with good epoxide selectivity. The catalytic performances in the “green” solvent are assigned to the formation of stable [C₁₂]₃[PW₁₂O₄₀] nanoparticle dispersions which have been characterized by transmission electron microscopy and dynamic and multiple light scattering experiments. Finally, the Kamlet–Taft parameters were measured in order to correlate the physicochemical properties of the solvents and the catalytic activity.