Synthesis of ordered mesoporous silica and alumina with controlled macroscopic morphologies

The ability to synthesize nanostructured inorganic materials with controlled microstructural and morphological features will provide materials with unique characteristics in unprecedented ways. This thesis investigates the synthesis of porous silica and alumina materials with controlled microstructures and desirable shapes using novel approaches based on template-assisted synthesis and chemical vapor deposition (CVD) techniques. It primarily focuses on fabricating mesoporous materials with unique microstructures and different morphologies (particles and membranes) and exploring the potential of the particle morphology in a polymer reaction application.; The template-assisted growth of mesoporous silica under acidic and quiescent conditions at an oil-water interface can generate mesostructured silica at the interface with fibrous, gyroidal, spherical, and film morphologies. Synthesis conditions can be used to alter the growth environment and control the product morphology. Fiber morphology is obtained at narrow range of experimental conditions due to slow and one-dimensional diffusion of silicon alkoxide through the interface. Variation in these conditions can alter the axial growth of silica and yield non-fibrous shapes. The fibers grow from their base attached to the interface and coalesce to form fibers with larger diameters. Gas transport in the mesoporous silica fibers is governed by combination of Knudsen and surface diffusion mechanisms. Surface diffusion contributes to 40% of the net flow reflecting a highly smooth pore surfaces. Real Knudsen and surface diffusivities are in the order of 10-3 and 10 -5 cm2/s respectively. The one-dimensional mesopores are 45 time longer than the macroscopic fiber length and align helically around the fiber axis, confirming the literature observations, with a pitch value of 1.05 micron.; For preparation of mesoporous silica materials as membranes, a novel counter diffusion self assembly (CDSA) approach is demonstrated. This approach, adopted from growth of silica fibers, introduces the precursors from the opposite sides of a ceramic supports and yields silica plugs grown within the support pores. Growth of defect-free silica membrane by this approach requires a hydrophobic support to enhance diffusion of the silicon alkoxide. According to gas permeation properties, silica plugs grow with thickness of ∼0.5 mm and have a mesoporous structure. Such mechanically strong membrane offers high potential in protein separation and polymer reaction applications.; Mesoporous membranes with controlled pore microstructure can be also obtained using CVD technique. Cyclic CVD modification of straight 20 nm pore alumina membranes demonstrates that variation of the precursor introduction scheme can affect the microstructure of alumina deposition within the support pores. Leaving residual of precursors in the support pore after introduction of each precursor causes deposition of alumina in a fractal structure suitable for gas separation applications. Purging the pore after each precursor, on the other hand, causes alumina to deposit in an atomic layer fashion to give cylindrical mesopores suitable for membrane reaction applications.; The use of ordered mesoporous materials supported with titanocene catalyst as nano-reactors for the extrusion of high quality polyethylene (PE) fibers is demonstrated. The steric effect of the straight 3 nm pores of mesoporous silica template the growth of 60 nm diameter PE fibers with extended-chain crystalline structures. The nascent polymer fibers aggregate into 1--30 mum microfibers which further aggregate into PE fiber bundles. Mechanical properties, measured for the first time, demonstrate that these fibers exhibit improved tensile strength compared to commercial PE fibers.