Nanostructured MnOx catalysts in the liquid phase selective oxidation of benzyl alcohol with oxygen. Part II. Reaction mechanism, kinetics and deactivation pattern.

The mechanism and kinetics of the liquid phase selective oxidation of benzyl alcohol to benzaldehyde with oxygen on bare and Ce or Fe promoted (Mnat/Meat, 3) MnOx catalysts [1] have been investigated. Irrespective of toluene as solvent, the bare and promoted catalysts drive the selective oxidation of the substrate via the Mars-van Krevelen redox mechanism due to a high reactivity of oxygen species present on the catalyst surface. The experimental 0th-order dependence on alcohol concentration signals the occurrence of a Langmuir-Hinshelwood (L-H) reaction pathway, kinetically controlled by adsorption-desorption steps (r.d.s.). Apparent 1st-order conversion trends reflect the occurrence of deactivation phenomena (i.e., parallel fouling) leading to a residual constant activity level during 50 h. DRIFT analyses of the fresh and used catalysts uncover the incipient formation of benzoic acid hindering the oxidation functionality by strong adsorption on active sites. Calcination at T>=473K restores the functionality of the used catalysts by the removal of strongly adsorbed benzoate species.