Cobalt-catalyzed oxidation of terpenes: Co-MCM-41 as an efficient shape-selective heterogeneous catalyst for aerobic oxidation of isolongifolene under solvent-free conditions

Cobalt-containing MCM-41 was used as a heterogeneous catalyst for the liquid-phase aerobic oxidation of isolongifolene, one of the most available sesquiterpenes. The material was prepared by direct incorporation of cobalt into the framework through the hydrothermal method, and was characterized by ICP-AES, N2 adsorption–desorption, TEM, XRD, SAXS, XPS, and H2-TPR techniques. Characterizations suggest that cobalt introduced into MCM-41 exists mainly in a single-site Co2+ state, whereas the amounts of the extraframework Co3O4 are relatively low. The oxidation of isolongifolene under mild solvent-free conditions results mainly (90% selectivity) in isolongifolen-9-one, a compound which occupies a vintage place in modern perfume industry. The total concentration of oxygenated products in the final mixtures reaches 90 wt%. The catalyst undergoes no metal leaching, and can be easily recovered and re-used. A silica-included cobalt catalyst prepared through a conventional sol–gel method shows a catalytic activity comparable with that of Co-MCM-41; however, selectivity is much lower.