Controlled Interactions between Anhydrous Keggin-Type Heteropolyacids and Silica Support: Preparation and Characterization of Well-Defined Silica-Supported Polyoxometalate Species

Anhydrous Keggin-type phosphorus heteropolyacids were deposited on partially dehydroxylated silica by using the surface organometallic chemistry (SOMC) strategy. The resulting solids were characterized by a combination of physicochemical methods including IR. Raman, ID and 2D H-1, and P-31 MAS NMR, electron microscopy experiments and density functional theory (DFT) calculations. It is shown that the main surface species is [ Si(OH center dot center dot center dot H-1)](2)[H+](1)[PM12O403 center dot center dot] where the polyoxometalate is linked to the support by proton interaction with two silanols. Two other minor species (10% each) are formed by coordination of the polyoxometalate to the surface via the interaction between all three protons with three silanol groups or via three covalent bonds formed by dehydroxylation of the above species. Comparison of the reactivity of these solids and of compounds prepared by a classical way shows that the samples prepared by the SOMC approach contain ca. 7 times more acid sites.