The support as a chemical reagent in the preparation of WOx/γ-Al2O3 catalysts:: Formation and deposition of aluminotungstic heteropolyanions

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 rationalized as a ligand-promoted oxide dissolution effect which is well-known in geochemistry: the ability of tungstates to complex dissolved aluminum causes a shift of the alumina dissolution equilibrium toward the formation of dissolved aluminic species. Mixed AI-W-O species can easily be identified in solution by means of Al-27 liquid-state NMR. We also demonstrate the deposition of aluminotungstic species onto the support during the preparation of WOx/Al2O3 catalysts by equilibrium adsorption. This means that the tungstic 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 Caillerie et al. J. Am. Chem. Sec. 1995, 117, 11471) and MoOx/Al2O3 catalysts (Carrier et at. J. Am. Chem. Sec. 1997, 119, 10137) imply that the alumina support can act as a real chemical reagent in the first steps of catalyst preparation.