Selective dehydroisomerization of cyclic monoterpenes to p-cymene over silica-supported CdO

Cadmium oxide supported on silica is a highly active and selective bifunctional catalyst for the clean synthesis of p-cymene using dehydroisomerization of cyclic monoterpenes such as alpha-pinene, beta-pinene, limonene, alpha-terpinene, gamma-terpinene, and terpinolene. The dehydroisomerization occurs via acid-redox bifunctional catalysis at the gas -solid interface involving isomerization of monoterpenes to p-menthadienes on acid sites of silica support fol-lowed by p-menthadiene dehydrogenation to p-cymene on oxo-metal sites of CdO. The less reactive bicyclic monoterpenes, such as alpha-pinene and beta-pinene, give 91-95% p-cymene yields at 325-375 degrees C, whereas more reactive monocyclic monoterpenes, such as limonene, alpha-terpinene, gamma-terpinene, and terpinolene, give a 100% yield at 200-250 degrees C. To the best of our knowledge, CdO/SiO2 has the highest efficiency in monoterpene-to-p-cymene dehydroisomerization among the catalysts reported so far. The catalyst was characterized by BET, TGA, XRD, DRIFTS, H2-TPR, and ICP-OES.