REACTIVITY OF MOLYBDENA WITH SILICA AND ALUMINA

Molybdena-alumina and molybdena-siljca systems have been prepared by impregnating the support with aqueous solutions of ammonium heptamolybdate (samples I) or by mixing MoO3 with Al2O3 or SiO2 (samples M), and calcining at 770 K (to decompose the molybdate) and 1100 K (to melt molybdena). In all eight cases the composition corresponds to two monolayers of molybdena. The solids have been characterized by nitrogen adsorption, X-ray diffraction, electronic and infrared spectroscopies, temperature programmed reduction and electron microscopy. Whichever the preparation method, only molybdena is detected on silica, and molybdena and Al2(MoO4)3 on alumina. The dispersion of molybdena, however, depends on the preparation method and the nature of the support. Formation of ternary Mo-Al-O compounds is due to support-supported phase interaction, that stabilizes molybdena even upon calcining at 1100 K, while MoO3 is removed by sublimation from the silica surface, leaving only 6.5% of the original loading. This behaviour can be related to the differences in the acidities of the supports, and then reaction of the more basic alumina with molybdena is easier than reaction with silica, thus accounting for the weaker interaction of this support with molybdena.