Aa. Said, MUTUAL INFLUENCES BETWEEN AMMONIUM HEPTAMOLYBDATE AND GAMMA-ALUMINA DURING THEIR THERMAL TREATMENTS, Thermochimica acta, 236, 1994, pp. 93-104
Different proportions of ammonium heptamolybdate (AHM) supported on ga
mma-alumina were prepared by the impregnation method. The thermal deco
mposition stages of AHM and the solid products were investigated using
DTA, TG, XRD and IR techniques. The results obtained revealed that th
e absence of all peaks accompanying the decomposition stages of AHM at
low loading levels (< 20 mol%) was due to the dispersion capacity of
the support. On increasing the % loading of AHM, gamma-alumina retarde
d the formation of the two intermediates, (NH4)4Mo7O23.2H2O and (NH4)2
Mo7O22.2H2O, while favoring the decomposition of the latter to produce
solid MoO3. The produced MoO3 readily interacted with Al2O3 above 500
-degrees-C to form Al2(MoO4)3 phase which became highly crystallized w
hen calcined at 700-degrees-C. The aluminum molybdate spinel formed wa
s thermally stable up to 800-degrees-C, and then decomposed above this
temperature into alpha-Al2O3 and MoO3. Moreover, the Al2O3 support de
creased the crystallinity of MoO3, whereas Al2(MoO4)3 only affected th
e surface-active site, Mo=O. Finally, the presence Of MoO3 greatly enh
anced the crystallization of alumina into the kappa- and alpha-phases
upon heating at 700 and 900-degrees-C, respectively.