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Free keywords:
Catalyst deactivation;
Alumina;
Epoxidation;
Hydrogen peroxide;
Solid-state NMR
Abstract:
This report addresses several questions regarding the deactivation of alumina in catalytic epoxidation using aqueous 70% H2O2. The structural, textural, morphological, and chemical changes of a polycrystalline alumina (γ-Al2O3 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-Al2O3 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 AlOH sites, which are active for catalytic epoxidation. In this way, the water in the alumina/H2O2 catalytic system seems to be significant in prolonging the lifetime of the catalyst.
