Synthesis and characterization of mesoporous niobium-doped silica molecular sieves

Hexagonally-packed mesoporous Nb-doped silica molecular sieves were hydrothermally synthesized with various siliceous sources and dopant concentration levels. The microstructure of mesoporous materials was characterized by a variety of physico-chemical methods. The pore packing order of the Nb-doped material depended strongly on the synthesis conditions such as aging temperature, pH of reaction mixture, surfactant-to-Si ratio, and dopant concentration. Different silica precursors gave rise to different reactivity with the Nb dopants and interactions with the surfactants. The pore size and surface area of the materials can be systematically varied. The chemical environment of the Nb dopants in the silica framework was studied by XPS, EPR, PA-FTIR, UV-Vis, and ²⁹Si MAS NMR. The results indicated that the Nb⁵⁺ ions were well dispersed in the framework of the mesoporous silica. The diffuse-reflectance UV- Vis spectrum of the mesoporous Nb-doped silica is red-shifted with respect to pure mesoporous silica. The ²⁹Si MAS NMR spectrum contained a broad component at about -105 ppm, which may have been attributed to the presence of Nb-O-Si bonding in the mesostructure.