Investigation Of Structural Stability Of Mesoporous Material MCM-41 Under High Temperature And High Pressure

Since the discovery of MCM-41 by the scientists from the Mobil Co. at 1992, mesoporous materials with unique properties, including large pore size, narrow pore size distribution and high surface area, have aroused a great deal of interests due to their potential to overcome the drawback of zeolite—limited diffusion of molecules due to the small channels. Among them, extensive researches have been performed on MCM-41 which possesses a hexagonal array of one-dimensional channels of uniform mesopores with pore diameter in the range 2～30 nm. So far, many papers have been published about the properties of MCM-41 such as catalysts and advanced materials. In the diverse applications of mesoporous material MCM-41, the structural stability is particularly important. Low hydrothermal stability of MCM-41 undoubtedly limits many of its applications, which is thought to can be improved by increasing the thickness of the pore walls and/or by enhancing the local ordering of the walls. From the thermodynamic point of view, mesoporous materials are members of unstable phase. With the change of pressure or temperature, a particular mesoporous material may be transformed into another mesoporous material, a microporous material or an inorganic salt. This implies that, in principle, recrystalization of the pore wall of MCM-41 to microporous material or more stable phase is possible under high-pressure and high-temperature conditions. But up to now, much less work has been performed about the mechanical stability of MCM-41, although many of its applications require it to be subjected to high-pressure and high-temperature. In this thesis, we have investigated the stability, phase transition and recrystallization of mesoporous material MCM-41 under high-pressure and high-temperature conditions using the Piston-Cylinder press, Walker MA6/8 superpress and LECO hydrothermal high-pressure and high-temperature apparatus.The low-angle X-ray diffraction (XRD) pattern suggested that mesoscopic order in these materials can be retained up to 10GPa at room temperature. The stability of MCM-41 decreased rapidly with the increasing of temperature, which will collapse completely at the temperature of 300℃and the pressure of 1.0GPa. The sequential phase transition with increase in pressure and temperature for mesoporous material MCM-41 is MCM-41→amorphous silica→α-quartz. More interesting is that MCM-41 transformed intoα-cristobalite under high-temperature, high-pressure hydrothermal condition (600℃, 0.15GPa).Post-synthesis recrystallization of MCM-41 has been used to prepare highly order MCM-41 material. It was found that the post-synthesis recrystallized MCM-41 possessed much better long rang structural ordering as observed by powder XRD and high resolution transmission electron microscopy (HRTEM). But obviously collapse of the port of MCM-41 was observed in the HRTEM images, which will greatly affected the application of MCM-41 in catalysis and seperation.