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This report addresses several questions regarding the deactivation of alumina in catalytic epoxidation using aqueous 70% H 2 O 2 . The structural, textural, morphological, and chemical changes of a polycrystalline alumina ( γ -Al 2 O 3 and boehmite) were studied in five consecutives reactions of 24 h. The chemical and physical processes involved in the transformations during alumina recycling are attributed mainly to the presence of water in the reaction mixture. Water plays a dual role in the catalytic system. On the one hand, water may cause deleterious changes of the structure of γ -Al 2 O 3 and its textural properties. On the other hand, the presence of water shifts the adsorption equilibria of the organic molecules (acetic acid, diols, and oligomers), preserving the type Ia Al OH sites, which are active for catalytic epoxidation. In this way, the water in the alumina/H 2 O 2 catalytic system seems to be significant in prolonging the lifetime of the catalyst.

This study answers several pending questions about alumina-catalyzed epoxidation with aqueous 70 wt% H 2 O 2 . To evaluate the effect of the water-to-aluminum tri- sec -butoxide molar ratio, this was systematically changed from 1 to 24. The xerogels were calcined at 450 °C and gave different γ -Al 2 O 3 's with distinct textural and acidic properties. A combination of 27 Al MAS NMR and TPD-NH 3 results of calcined aluminas allowed us to assign the type Ia Al OH sites as the catalytic sites for epoxidation. The type Ib Al OH sites have no function in catalytic epoxidation, because ethyl acetate poisons these sites. The strong acid sites of types IIa, IIb, and III Al OH groups are responsible for the undesired H 2 O 2 decomposition and decreased oxidant selectivity.

Well-ordered [Nb]-MCM-41 was synthesized at room temperature with essentially all niobium present in the sample substituted into the silica framework. The material was calcined and then silylated using hexamethyldisilazane (HMDS). Both calcined and silylated materials were characterized by X-ray diffraction (XRD), diffuse reflectance UV–vis, inductively coupled plasma emission (ICP-OES), infrared spectroscopy (FTIR), N2 adsorption at low temperatures, thermogravimetry (TG), and nuclear magnetic resonance of 29 Si with magic-angle spinning (NMRMAS). The materials were used as catalysts in the epoxidation of cis-cyclooctene with tert-butyl hydroperoxide (TBHP) and 70 wt% aqueous hydrogen peroxide as oxidants, to verify the influence of the silylation on the activity and selectivity. It was observed that the silylated material was more active for both oxidants, reaching 62% conversion and 94% selectivity after 48 h with TBHP and 13% conversion and 80% selectivity after 5 h with aqueous hydrogen peroxide. © 2006 Elsevier Inc. All rights reserved.

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 nm 2 , 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.

The structural, textural, and chemical properties as well as the hydrophilicity of four different sol–gel aluminas were determined by different methods and compared with their activity in the catalytic epoxidation of cis-cyclooctene with aqueous H2O2. It was found that the hydrophilicity of the γ-alumina surface is the principal factor responsible for the difference in catalytic activity. The hydrophilicity of γ-alumina should be sufficient to guarantee a fast reaction with hydrogen peroxide on the surface but should not to be too high in order to allow the approach of olefin to the active sites.

At 100°C cyclohexanol and cyclohexanone, obtained in the TS-1 catalyzed oxidation of cyclohexane, are further oxidized in uncatalyzed and TS-1 catalyzed reactions. Cyclohexanol is very selectively oxidized to cyclohexanone inside the porous system of TS-1 and unselectively oxidized to several oxidation products on the external surface. This unselective oxidation can be suppressed by the addition of 2,6-di-tert-butyl-4-methylphenol (BHT), which does not enter the molecular sieve pore system and which efficiently reduces uncatalyzed oxidation. Cyclohexanone oxidation is mostly uncatalyzed, forming predominantly dicarboxylic acids, and is not influenced by BHT. The products of the TS-1 catalyzed cyclohexanone oxidation are partially retained in the porous system, thus explaining the deactivation of the catalyst. Comparison with other systems shows that the turnover frequency of the TS-1 catalyzed cyclohexane oxidation is very similar to that of the radical-chain process, thus suggesting that the rate-determining step of the TS-1 catalyzed reaction is also the homolytic cleavage of a C-H bond.