Solvent Effects during the Oxidation of Olefins and Alcohols with Hydrogen Peroxide on Ti-Beta Catalyst: The Influence of the Hydrophilicity–Hydrophobicity of the Zeolite

It is presented in this paper that the hydrophilic–hydrophobic characteristics of Ti-zeolites play a very important role in their reactivity for liquid phase oxidations using H2O2and organic hydroperoxides. To show this, the influence of the nature of solvent on the reactivity and selectivity features of the large pore Ti-beta catalyst has been studied during the oxidation of 1-hexene and cyclohexanol, as representative substrates, using aqueous hydrogen peroxide as oxidant and under single liquid phase conditions. It has been shown that the reactivity of Ti-beta was highest in acetonitrile, a polar and nonprotic solvent, for both 1-hexene and cyclohexanol oxidations. In contrast, the reactivity of the medium pore TS-1 for 1-hexene oxidation was seen to be much higher in methanol (protic) than in acetonitrile (aprotic) solvent. These differences have been related to the hydrophilic character of Ti-beta, in contrast to the hydrophobic character of TS-1. The selectivities to the epoxide and to the ketone during 1-hexene and cyclohexanol oxidations, respectively, on Ti-beta were also highest in acetonitrile. This was associated to the poisoning of the Brønsted acid sites associated with framework aluminum by the basic acetonitrile molecules, thus avoiding acid-catalyzed secondary reactions. The different effects that solvents have on the catalytic properties of Ti-beta and TS-1 are of prime importance from the point of view of the application of these catalysts, since, as can be seen from this work, the optimum operating conditions would be quite different for both the Ti-beta and the TS-1 materials.