Zeolite-encapsulated vanadium picolinate peroxo complexes active for catalytic hydrocarbon oxidations

Zeolite-encapsulated vanadium (IV) picolinate complexes were prepared by treatment of dehydrated VO(2+)–NaY zeolite with molten picolinic acids. Treatment of the NaY-encapsulated VO(pic)₂ complex with urea hydrogen peroxide adduct in acetonitrile allowed to generate peroxovanadium species. The structure of vanadium peroxo species was studied by UV–vis, Raman and XAFS spectroscopies which suggested the formation of monoperoxo monopicolinate complex which could be active intermediate for various oxidation reactions with the catalysts. To elucidate effect of the encapsulation on catalytic performance, the catalytic properties of the encapsulated complexes were compared with that of corresponding homogeneous catalyst H[VO(O₂)(pic)₂]·H₂O. The novel `ship-in-a-bottle' catalysts retain solution-like activities in aliphatic and aromatic hydrocarbon oxidations as well as in alcohol oxidation. In addition, the encapsulated vanadium picolinate catalysts showed a number of distinct features such as preferable oxidation of smaller substrates in competitive oxidations, increased selectivity of the oxidation of terminal CH_3⁻ group in isomeric octanes and preferable (sometimes exclusive) formation of alkyl hydroperoxides in alkane oxidations. The distinct features were explained in terms of intrazeolitic location of the active complexes that imposed transport discrimination and substrate orientation. On the basis of the experimental data, a possible mechanism was discussed. Stability of the vanadium complexes during the liquid phase oxidations and leaching from the NaY zeolite matrix were also examined.