PSEUDOTETRAHEDRAL O3 2V=0 CENTERS IMMOBILIZED ON THE WALLS OF A MESOPOROUS, CUBIC MCM-48 SUPPORT - PREPARATION, CHARACTERIZATION, AND REACTIVITY TOWARD WATER AS INVESTIGATED BY V-51 NMR AND UV-VIS SPECTROSCOPIES/

The high hydroxyl content of mesoporous silicates makes it possible to use them as anchors for the attachment of transition-metal species. I n the present study, three different samples corresponding to atomic V /Si ratio of 0.01, 0.05, and 0.10 have been prepared by reacting a sil ica-based mesoporous matrix of the cubic MCM-48 structure, with dry he xane solutions of O=V(OiPr)(3). Bulk structural characterization (X-ra y diffraction and BET) shows that the cubic structure is maintained du ring the impregnation process. In the presence of V centers, a noticea ble decrease of the main-pore diameter and of the a unit-cell paramete r is observed. Thus, a strong interaction between the mesoporous walls and the V centers is suggested. The microstructural configuration of V centers has been investigated further by V-51 NMR and UV-vis spectro scopies. Both techniques reveal that, in the absence of water, pseudot etrahedral O3/2V=O centers, probably coordinated by three (Si-O-V) bri dges, are grafted to the mesoporous walls. The presence of these funct ional groups and their particular reactivity toward entering ligands s uch as water impart a number of unique properties to the bulk V/MCM-48 , including color changes from white to orange in the presence of mois ture.