In situ DRIFT study of the reactivity and reaction mechanism of catalysts based on iron-molybdenum oxides encapsulated in Boralite for the selective oxidation of alkylaromatics

An in situ DRIFT investigation of the behavior of iron-molybdenum oxides en capsulated in Boralite (FeMo/Bor) during the oxidation of toluene is report ed. The study was carried out to obtain a better understanding of the diffe rences between this catalytic material and (i) V-TiO2 based catalysts and ( ii) bulk Fe-2(MoO4)(3). V-TiO2 based catalysts show a severe decrease in th e selectivity to benzaldehyde with increasing conversion of toluene, in con trast FeMo/Bor samples. The effect was attributed to the presence of strong er Lewis acid sites in vanadium-based catalysts which, activating the carbo n atom of the carbonyl groups, facilitate its nucleophilic attack to form b enzoate species which further degrade to carbon oxides. FeMo/Bor shows high er selectivity at low conversion than bulk Fe-2(MoO4)(3), probably due to t he presence of nanosized iron-molybdate particles inside the zeolite channe ls, and lower selectivity at high conversion. Due to back-diffusion limitat ions inside the zeolite pores, the aromatic ring of the alkylaromatic is ox idatively attacked to form maleic anhydride, precursor of the further oxida tion to carbon oxides. In FeMo/Bor a different main pathway is responsible for the lowering of selectivity at high conversion with respect to V-TiO2 b ased catalysts. (C) 2000 Elsevier Science B.V. All rights reserved.