On the paradox of transition metal-free alumina-catalyzed epoxidation with aqueous hydrogen peroxide

The aluminas obtained by calcination of nanocrystalline boehmite, prepared by the sol-gel method, in the range 200-1000 °C were characterized by powder X-ray diffraction, thermogravimetric analysis, Fourier transformed infrared spectroscopy, nitrogen adsorption-desorption, and scanning electron microscopy. The catalytic activity of these aluminas in the epoxidation of cis-cyclooctene with aqueous 70 wt% hydrogen peroxide shows, after 24 h, a profile with two maxima at 400 and 700 °C that corresponds to 85 and 80% yield, respectively. However, this profile cannot be directly related to the structural and textural properties of the calcined aluminas. Regarding the hydrophilicities of the calcined aluminas, determined by the amount of water molecules per nm2, the epoxide yield of the aluminas can be segregated into three groups corresponding to the general crystallographic classification of these aluminas: boehmite, γ-series, and δ-series. The aluminas from the γ-series show the highest cyclooctene oxide yield after 1 h; however, due to surface modifications occurring during the reaction, the aluminas from the δ-series also give good yields after 24 h. The water adsorption capacity or hydrophilicity of these aluminas shows a strong correlation with epoxide productivity of the calcined aluminas at 24 h. Generally, better epoxide productivity is observed for lower hydrophilicities. The hydrogen peroxide efficiencies are typically 2.0-2.5 mmol of oxidant per mmol of epoxide formed after 24 h. However, the aluminas of the δ-series show poor efficiency at the beginning of the reaction that improves from an average of 4.1 to 2.3 mmol of oxidant per mmol of epoxide after 2 h due to modification of their surfaces.