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)center dot 6H(2)O (M = Mg, Zn, Ni) and Al(NO3)(3)center dot 9H(2)O 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 degrees 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. (c) 2006 Elsevier Inc. All rights reserved.