ESR and solid state MAS NMR study of the silica-supported H3+nPV12-nO40 (n=0,1,2,3) heteropolyacids

The influence of the high surface area silica support on the thermal stability of the H3+nPVnMo12-nO40 heteropolyacids (HPA) has been investigated by means of ESR and solid state MAS NMR techniques. The sequence of Mo(V) and V(IV) ESR signals, as well as NMR of 31P and 51V, allow one to follow the dehydration/destruction processes in HPA. Deposition on silica lowers the HPA stability, but upon contact with humid air at room temperature a rapid reconstruction of the Keggin anions from the final decomposition products occurs. It is argued that under 'moist' conditions the hydrophilic surface of silica provides a quasi-liquid surface layer faiclitating the reaction. Thus, only data obtained in a water-free atmosphere can provide reliable information about the degree of HPA decomposition. Previous controversy over stability of silica-supported HPA is attributed to the neglect of this factor. (author). 30 refs.; 7 figs