Solution-Phase Grafting of Titanium Dioxide onto the Pore Surface of Mesoporous Silicates:  Synthesis and Structural Characterization

Titanium dioxide, a large-bandgap semiconductor and versatile photocatalyst, has been grafted onto the pore surface of MCM-41 and FSM-16 (a mesoporous material derived from kanemite) by reacting TiCl₄ in hexanes with the as-synthesized mesostructured silicate. The products have been extensively characterized by powder XRD, TEM, SEM, EDS, XPS, N₂ adsorption, SANS contrast matching, solid-state ¹H MAS NMR, IR, and UV−vis spectroscopies. It was found that titania forms well-dispersed isolated (TiO₂)_n clusters (n 30−70) within the channel structure. These are attached to the silicate walls via Si−O−Ti bonds. A minor second phase consisting of anatase crystallites ca. 100−250 Å in diameter on the external surface of the mesoporous silicate crystals was sometimes obtained. It is concluded that an organic moiety, such as the surfactant present in the pores, or a physical constraint, such as the pore walls, is necessary to prevent the creation of large TiO₂ agglomerates and enable the formation of nanosized TiO₂ clusters. The titania-grafted MCM-41 samples exhibited good catalytic activity for photobleaching of rhodamine-6G and for oxidation of α-terpineol; however, product selectivity was low.