The modification of the pore surfaces of ordered porous silicates using inorganic oxides

The first report of MCM-41 in 1992 opened the door to ordered, mesoporous silicates that can be readily synthesized from a variety of silica sources and surfactants in a low temperature procedure. Simple adjustments to the synthesis can be used to create a variety of morphologies and pore shapes in the final ordered product. The high surface area, high pore volume, and narrow pore size distribution make mesoporous silicates appealing for such applications as catalysis, ion-exchange, and microelectronics. In order to exploit this potential, new techniques were developed for the modification of ordered porous silicates with titania and manganese oxides. Titania nanocrystals approximately 12–16 Å in diameter were grown on the surfaces of MCM-41 and FSM-16. By using mesostructured silicates from which the surfactant had not yet been removed, the hydrolysis of TiCl₄ to TiO₂ was controlled, preventing the formation of large titania agglomerates. A wide variety of diffraction, sorption, and spectroscopic techniques were used to demonstrate the formation of well-dispersed anatase nanoclusters chemically bound to the silicate surface and the systematic manipulation of optical properties of the grafted titania. It was also found that the mesoporous framework was unharmed by the modification process. Testing of Ti-MCM-41 found it to be a viable catalyst for the thermally activated decomposition of large organic molecules.; Because the methods currently available for the synthesis of the electrochemically active birnessite were not amenable to the modification of an ordered silica surface, a milder synthesis for Mg-birnessite and chalcophanite was developed that could be adapted to a grafting technique. The manganese oxides were analyzed using powder XRD, TGA, SEM, and NMR and IR spectroscopies. Electrochemical studies determined that these layered materials had a higher lithium intercalation capacity and greater stability than other manganese oxides, making them potentially useful battery materials. Application of the new synthesis to a grafting procedure produced Mn-modified materials which were thoroughly characterized and found to consist of agglomerates of manganese oxide nanoclusters occluded within the silicate channels. These materials had a high ion exchange capacity. However, difficulties in the electrochemical analysis prevented complete determination of the lithium capacity and stability.