Hydrotalcites composition as catalysts: Preparation and their behavior on epoxidation of two bicycloalkenes

Hydrotalcites containing heptamolybdate (Mo7O246-) in the interlayer and Al combined with different divalent cations (Mg, Zn, Ni) in the brucite-like layers were prepared by ion exchange from suitable precursors containing terephthalate. To prepare these materials, a given amount of terephthalic acid was adjusted at pH 8.0 with NaOH and then mixed with a second solution, containing amounts of M(NO3)2 · 6H2O (M = Mg, Zn, Ni) and Al(NO3)3 · 9H2O corresponding to a M2+/M3+ molar ratio of 2, keeping the molar Al:terephthalate ratio to a value of 2:1. After refluxing under inert atmosphere and drying in vacuum, hydrotalcites were exchanged with aqueous solutions of ammonium heptamolybdate for an Al/Mo molar ratio of 4:1. Characterization of these solids was made using element chemical analysis, adsorption–desorption isotherms of nitrogen at −196 °C, powder X-ray diffraction, and FT-IR spectroscopy. The catalysts were tested in liquid phase oxidation of bicyclo[2.2.1]-2,5-heptadiene (norbornadiene); benzobicyclo[2.2.1]-2-heptene (benzonorbornadiene) and cyclohexene using hydrogen peroxide and dioxane or dioxane–butyl maleate mixture as solvents. The selectivity to epoxides was found to depend on the hydrotalcite composition and on the reaction solvent. Hydrotalcites containing Ni/Al–Mo and Zn/Al–Mo were more selective than the Mg/Al–Mo one. The replacement of dioxane with a dioxane–butyl maleate mixture led to a decrease in the selectivity to epoxide. Recycling of the catalysts over six successive runs demonstrated their stability.