Pseudotetrahedral O3/2VO Centers Immobilized on the Walls of a Mesoporous, Cubic MCM-48 Support:  Preparation, Characterization, and Reactivity toward Water As Investigated by51V 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. In 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 silica-based mesoporous matrix of the cubic MCM-48 structure, with dry hexane solutions of OV(OiPr)3. Bulk structural characterization (X-ray diffraction and BET) shows that the cubic structure is maintained during the impregnation process. In the presence of V centers, a noticeable decrease of the main-pore diameter and of the a unit-cell parameter 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 51V NMR and UV−vis spectroscopies. Both techniques reveal that, in the absence of water, pseudotetrahedral O3/2VO centers, probably coordinated by three (Si−O−V) bridges, are grafted to the mesoporous walls. The presence of these functional groups and their particular reactivity toward entering ligands such 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 moisture.