X. Carrier et al., The support as a chemical reagent in the preparation of WOx/gamma-Al2O3 catalysts: Formation and deposition of aluminotungstic heteropolyanions, J AM CHEM S, 121(14), 1999, pp. 3377-3381
We show in this study that suspending alumina in a tungstate solution leads
to the formation of Keggin-type aluminotungstic species in solution. This
process occurs even at "nonaggressive" (near-neutral) pH. This can be ratio
nalized as a ligand-promoted oxide dissolution effect which is well-known i
n geochemistry: the ability of tungstates to complex dissolved aluminum cau
ses a shift of the alumina dissolution equilibrium toward the formation of
dissolved aluminic species. Mixed AI-W-O species can easily be identified i
n solution by means of Al-27 liquid-state NMR. We also demonstrate the depo
sition of aluminotungstic species onto the support during the preparation o
f WOx/Al2O3 catalysts by equilibrium adsorption. This means that the tungst
ic species deposited on the support are not always isopolyanions, contrary
to what is suggested in most of the previous studies on this type of system
. This study as well as previous studies on M(II)/Al2O3 (d'Espinose de la C
aillerie et al. J. Am. Chem. Sec. 1995, 117, 11471) and MoOx/Al2O3 catalyst
s (Carrier et at. J. Am. Chem. Sec. 1997, 119, 10137) imply that the alumin
a support can act as a real chemical reagent in the first steps of catalyst
preparation.