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Solid-solid interaction in the pure and Li2O-doped MoO3Al2O3 system

1996; Elsevier BV; Volume: 275; Issue: 2 Linguagem: Inglês

10.1016/0040-6031(95)02694-0

1872-762X

G.A. El-Shobaky, F.H.A. Abdalla, A.M. Ghozza, Khail Khalil,

Catalysis and Hydrodesulfurization Studies

The effects of Li2O doping (1.5 and 6 mol%) on solid-solid interactions and the phase transformation process in the MoO3Al2O3 system were investigated using thermogravimetry, differential thermal analysis and X-ray diffraction (TG, DTA and XRD) techniques. The proportions of molybdena expressed at wt% MoO3 were 22 and 41.4. The results obtained revealed that MoO3 interacted with Al2O3 at temperatures starting from 500°C, forming orthorhombic Al2(MoO4)3; doping with lithium oxide was found to promote the reaction. The complete transformation of molybdenum trioxide into aluminum molybdate required heating of the mixed oxides at 700°C. The molybdate produced decomposed at temperatures above 800°C yielding α-Al2O3 and MoO3. The Li2O doping enhanced the crystallization of α-alumina and retarded the thermal decomposition of Al2(MoO4)3. The MoO3 produced partly sublimed and the remaining portion dissolved in the alumina matrix forming an MoO3Al2O3 solid solution. Li2O doping increased the amount of MoO3 sublimed at 900–1100°C to an extent proportional to the concentration of the dopant. In other words, Li2O treatment decreased the solubility of MoO3 in Al2O3. The promoting effect of Li2O on Al2(MoO4)3 formation at 500°C was attributed to dissolution of a small portion of Li2O in the MoO3 lattice, with subsequent increase in the mobility of Mo6+ ions. The possible increase in the mobility of these ions might also account for the observed decrease in the solubility of MoO3 in Al2O3.