MUTUAL INFLUENCES BETWEEN AMMONIUM HEPTAMOLYBDATE AND GAMMA-ALUMINA DURING THEIR THERMAL TREATMENTS

Authors
Citation
Aa. Said, MUTUAL INFLUENCES BETWEEN AMMONIUM HEPTAMOLYBDATE AND GAMMA-ALUMINA DURING THEIR THERMAL TREATMENTS, Thermochimica acta, 236, 1994, pp. 93-104
Citations number
20
Categorie Soggetti
Chemistry Analytical
Journal title
ISSN journal
00406031
Volume
236
Year of publication
1994
Pages
93 - 104
Database
ISI
SICI code
0040-6031(1994)236:<93:MIBAHA>2.0.ZU;2-9
Abstract
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.