H2O2-based allylic oxidation of α-pinene over different single site catalysts

Catalytic properties of different solids, containing Ti(IV), Zr(IV), and Fe(III) isolated in inorganic matrixes, have been assessed in allylic oxidation of α-pinene with aqueous H2O2. Among the catalysts studied were mesostructured metal silicates Ti-MMM-2 and Fe-MMM-2, TiO2-SiO2 mixed oxides, Ti and Zr grafted on mesoporous silica cellular foams (Ti- and Zr-MCF), and a new composite material, H5PW11TiO40/silica, prepared by a sol-gel method. The catalysts were characterized by elemental analysis, N2 adsorption, and DRS-UV. The Ti- and Zr-containing materials showed catalytic properties superior to those of Fe-MMM-2. The reaction selectivity strongly depended on the solvent properties, substrate/oxidant molar ratio, reaction temperature, and α-pinene conversion. In optimized reaction conditions (MeCN, 30 °C), the total selectivity toward verbenol/verbenone reached the maximal value of 80-82% at 8-15% conversion. The H5PW11TiO40/silica catalyst can be used repeatedly without loss of activity and selectivity. No leaching of the active species occurs, and the oxidation is a true heterogeneous process operating via a radical nonchain mechanism.