Hydrothermal transformation and characterization of porous silica templated by surfactants

The effect of hydrothermal conditions on the interactions between surfactant molecules and silicate species plays a key role in the structural transformations of silica precursors. In this work the structures of different siliceous products obtained at various synthesis temperatures have been characterized by powder X-ray diffraction (XRD), FT-IR spectroscopy, Al-27 and Si-29 magic angle spinning (MAS) NMR, N-2 adsorption/desorption measurements, and thermogravimetric analysis. The XRD) results show that the removal of other soluble ions from synthesis mixture is a primary step for preparation of the mesostructure in the CTAB-silicate-water synthesis system. Increasing the hydrothermal temperature may lead to the various structures of porous silica and change the behaviors of surfactants in the synthesis mixture. With the increase of hydrothermal temperature from 100 degrees C to above 165 degrees C or the prolonging of reaction time at a fixed temperature of 170 degrees C, the transformation sequence of the following structural phases is identified by XRD: hexagonal MCM-41 --> lamellar M41S --> ZSM-5. Comparison with the results from XRD, pore structure measurement, FT-W, and NMR demonstrates that the intermediate product from the hexagonal mesostructure to microporous crystallite is the lamellar phase rather than the ill-defined hexagonal structure. DTG results indicate that the formation of different structures could result from the different surfactant templating molecules. The main contribution of this work hes in attempts to provide an effective route for controlling conditions of the formation of different mesoporous phases and to understand the cooperation of surfactant molecules and inorganic species.