High-temperature Synthesis Of Ordered Mesoporous Silicas And Understanding Of Their Synthetic Mechanisms

Ordered mesoporous silicas (OMSs) have been widely used in many fields such as ion-exchange, adsorption and separation, and industrial catalysts because of their high surface area, large adsorption capacity, diverse morphologies and tunable mesopores since the discovery of the M41S family by researchers at Mobil. As one of the hotspots in current research nowadays, a series of OMS have been increasingly reported including novel synthesized routes,all kinds of synthesized conditions,direct synthesis of a variety of morphologies (microball, fiber, film etc) and so on. Generally, the main research efforts have focused on the systems at mild temperature (less than 120℃), while fewer works have been reported for high temperatures (above 150℃), which are traditionally regarded as unfavorable condition for the formation of OMSs due to the distortion of micelles or the decomposition of hydrocarbon surfactants at such high temperatures. The low synthetic temperatures usually result in imperfectly condensed mesoporous walls with large amounts of terminal hydroxyl groups, which make the mesostructure unstable. It can be expected that the level of silica condensation will be directly enhanced by increasing the crystallization temperature.In this paper, Ordered mesoporous silicas (SBA-15 or MCM-41) have been directly hydrothermally synthesized at high temperatures (160–180℃) by using a solo hydrocarbon surfactant (P123 or CTAB). Based on the results and data of the experiment, we proposed that the surfactant micelle plays an important role during the cooperative self-assembly of the silicates and surfactants in the initial stage for the formation of basic mesostructure, and it is no longer indispensable in the subsequent hydrothermal treatment at high temperature.It is worth noting that when the temperature reaches extra high values, the walls would collapse because the walls could not withstand the high pressure under hydrothermal conditions.The work here discussed synthetic mechanisms of ordered mesoporous silicas from solo hydrocarbon surfactants at high temperature in detail, which provides a background for high-temperature synthesis of ordered mesoporous silicas.