Biomimetic Synthesis And Characterization Of The Mesoporous Silica Membranes

The ordered mesostructured membrane materials owing to their superior properties and great potential applications are becoming research focus in the material chemistry field. Mesoporous inorganic membranes with pore diameter of 2 50 nm not only have excellent thermal and chemical stability that inorganic membranes possess but have regular arrays of uniform channels and the dimensions can be tailored. The ordered mesoporous inorganic membrane is a promising material for membrane-based separation processes, catalysts, and chemical sensors, etc.Biomimetic synthesis has now become a promising field in inorganic materials chemical research. Biomimetic synthesis inspired by the biomineralization involves the controlled formation of inorganic materials with organic assembly as template, and the production of inorganic materials with special microstructure or mesostructure and excellent physical and chemical properties.The main works of this thesis are oriented mesoporous silica membranes are synthesized at air-water and solid-water interfaces by a surfactant-based supramolecular templating technique and characterized by XRD SEM TEM TGA N₂ adsorption and gas permeation experiment. The main results are as follows:1. Hexagonal mesostructured silica membranes have been synthesized at air-water interfaces by a surfactant hexagonal liquid crystal templating technique under quiescent aqueous acidic conditions. The influencial factors on membrane synthesis were investigated by XRD SEM and TGA studies. The growth of as-synthesized membrane involves two stages-the induction period and the subsequent rapid growth of a free-standing membrane. The induction period is dependent upon the concentration of the acid and observed induction period became longer when solution acidity reduced. And low acidity favors the growth of thick membrane. CTAB crystal diffraction peaks are observed in XRD pattern of as-synthesized membrane when surfactant concentration is high in solution, indicating that rich surfactant is in as-synthesis membrane layer. The influences of agitation intensity on crystal phase andsynthesis time on mesostructured order are observed by XRD studies. As-synthesized membrane goes through a transition from disorder to order and then to order reduced. SEM images show that superior surface of the free-standing membrane was smooth, however, inferior surface of it was gathered with rod-like particles. It was concluded that the membrane nucleated around hemi-micelles of surfactant located at the air-water interface and grew by coaggregation of the surfactant and silica into the water phase. It can also be estimated that the average thickness of the membrane is about 20 jam. TGA datas show that the decomposition of the surfactant in air involves three steps, and the total weight loss was about 65%.2. Mesoporous silica MCM-48 membranes have been synthesized on a coarse-pore a-AI2O3 ceramic tube through interfacial silica-surfactant self-assembly process. The membranes and powders were characterized by XRIX SEM^ TEM> TGAx N2 adsorption and gas permeation experiment. X-ray diffraction (XRD) results showed the membrane possessed a periodic mesostructure. The N2. adsorption and desorption isotherms also showed that the membrane was a typical mesoporous material with pore channel size of about 2.74 nm. The TGA experiment proved that the removal of the surfactant is a stepwise mechanism. The permeability of the calcined mesoporous MCM-48 membrane was evaluated by the permeation of single gases (H2 and N2) with pressure drops across the membrane of 40 240 kPa. The permeation of these gases through the calcined MCM-48 membrane was strongly governed by Knudsen diffusion. The permeance of the single gas was independent of the pressure drop, indicating that there was no contribution of viscous flow to the total permeation. This result supports that there exist no large pinhole and crack in the mesoporous MCM-48 membrane, in agreement with SEM observation.