Influence of alumina dissolution on the final state of MoOx/Al2O3 catalysts

This study shows that alumina dissolution controls the speciation of molybdic species deposited on the alumina surface during MoOX/Al2O3 catalysts preparation by the Incipient Wetness method. One hour after the start of the aqueous impregnation step we can easily identify the formation of bulk (NH4)(3)[Al(OH)(6)Mo6O18] with XRD. This is a clear evidence that there is an extensive reaction of molybdate ions with dissolved aluminum atoms during the preparation procedure. This phenomenon was previously demonstrated in the preparation of MoOX/Al2O3 catalysts by the Equilibrium Adsorption method, and we show here that this occurs also during Incipient Wetness impregnation. In this latter case, the aluminomolybic species deposited on the support is not dispersed at the molecular level but precipitates as a bulk species. The formation of this species can be avoided by a fast impregnation step followed by freezing the solid in liquid nitrogen and drying it under vacuum (freeze-drying), We also show that the nature of the catalyst obtained after calcination at 400 degrees C is dependent on the nature of the molybdic species obtained after the initial aqueous impregnation step. For the catalysts obtained by the classical Incipient Wetness impregnation, we can identify bulk MoO3 by XRD for loadings as low as 7 Mo wt %. For the catalysts obtained by freeze-drying, the formation of bulk MoO3 can be avoided for loadings as high as 12 Mo wt %.