Étude par RMN MAS liquide de $^{31}$P de l'évolution des solutions d'acide 12-tungstophosphorique (ion de Keggin) en fonction du pH, de la dilution et du temps

Heteropoly acids of the Keggin type H$_{\rm 8-x}$XM$_{12}$O$_{40}$, where X is the central atom (Si, P,...), x its oxidation state and M, a transition metal (Mo, W,...) are well known as oxidation and acid catalysts. The chief disadvantage of heteropoly acids is their weak thermal stability, resulting in a degradation of the active phase. Therefore, several attempts were taken on to stabilize them by supporting on various carriers. Silica is an excellent support for the heteropoly acids. Generally, the solid is prepared by impregnation of the active phase by a solution. In this paper, we proposed to determine and quantify the different tungstophosphate species by choosing the optimal conditions to incorporate one of such species. Results obtained by $^{31}$P NMR in aqueous medium of tungstophosphate species were discussed by varying the pH, the dilution and the evolution duration