Synthesis And Characterization Of Fluorescence Mesoporous Materials And Highly Steam-stable Mesoporous Molecular Sieves

Mesoporous molecular sieves show great potential applications and attract much attention because of their uniform and larger mesopores, in contrast, zeolite cannot effectively deal with large molecules due to the limitation of the micropore size. Ordered mesoporous materials with large pore size, high surface area, and thermal stability have been paid much attention in the fields of catalysis,adsorption, separation, and nanotechnology. Recently, mesoporous materials have been extended to optical applications. But large amount of optical mesoporous materials are synthesized from a post-treatment, and only a few examples are prepared from direct-condensation by fluorescent-functional silanes, mainly focused on fabrication of films. The post-treatment of mesoporous materials may result in blocking of some pores by fluorescent compounds and surface area of these materials is reduced significantly.In this work, we show one-pot synthesis of fluorescent mesoporous materials from Schiff base-functional silane. For example, a fluorescent hexagonal mesoporous silica has been successfully prepared by one-pot hydrothermal synthesis from TEOS and 2-((3-(triethoxysily)propylimino)methyl)phenol in the presence of CTMABr surfactant. After removal of surfactant in ethanol solution, the mesoporous silica still exhibits uniform mesopores, high BET surface area, large pore volume, and good fluorescent properties. Very importantly, this sample obtained could be used to detect Be²⁺ ion in water at neutral condition. More importantly, peak intensi in fluorescent spectra of extracted F-MCM-41 samples are dependent on the concentration of Be²⁺ ions in solutions, which is potentially important for detection of the presence of Be²⁺ ion.It is well known that Be²⁺ has very strong toxicity, and its content in drinking is strictly limited. For example, Be²⁺ ions cannot be more than 2μg/L in China, whose concentration is just in the range from 1 to 20μg/L for the detection in this work. Here, we present a facile and rapid method for detection of the presence of Be²⁺ ion by using fluorescent meosporous solids.Compared with microporous zeolite, mesoporous materials show poor hydrothermal stability, which seriously limit their extensive uses. The weaker hydrothermal stability of mesoporous materials is attributed to the amorphous nature of their pore wall. Therefore, it could be expected that the hydrothermal stability and activity of mesoporous materials be improved if zeolite structure units were itroduced into their amorphous wall. And if the temperature could be increased in the synthesis process, the stability of mesoporous materials will be improved.In the previous work, by using the zeolite"seed solution"to introduce the zeolite primary structure units into the walls of the mesoporous materials many mesostructured materials (MS) with better hydrothermal stability can be obtained. The evidence that proved the walls containing the structure units of zeolite are not direct including the increase of microporocity, because the mesoporous materials synthesized under low temperature contain microporous volume themselves.And almost all mesostructured materials are prepared at room temperature or relatively low temperatures (< 140°C). Low synthetic temperatures result in imperfectly condensed mesoporous walls with large amounts of terminal hydroxyl groups which make the mesostructure unstable, especially under hydrothermal or steam conditions. It can be expected that increasing the crystallization temperature will enhance the level of silica condensation. However, ordered mesoporous materials can not be synthesized at higher temperature in general cases. This is because surfactant molecules will not be able to direct mesoporous structure formation due to unfavourable conditions for micelle formation at the higher temperatures. In some cases, the large-chain surfactants will even decompose at more than 150°C. In ordered to prepare ordered mesoporous materials at high temperature, high temperature-resistant fluorocarbon surfactant (FC-4) is used together with troblock copolymer as"co-template"to prepare mesoporous silica at 150-200°C. In this way, 2-D hexagonal (p6mm) JLU-20 and 3-D body-cetered cubic (Im3m) JLU-21 are prepared.In our work mesoporous silica (JLU-20-S) is successfully synthesized at high-temperatures (180-220°C) by an assembly of preformed silicalite-I zeolite nanoclusters with the mixture of triblock copolymer surfactant (P123) and fluorocarbon surfactant (FC-4), which shows extraordinary stability in steam (800°C, 4 hours), compared with other mesoporous silica materials (JLU-20 synthesized at 190°C, MPS-9 prepared from preformed silicalite-I zeolite nanoclusters at 100°C, and SBA-15). The results of X-ray diffraction and transmission electron microscopy show that JLU-20-S has a hexagonal mesoporous symmetry, and the data obtained from N2 isotherms show that JLU-20-S contains both mesopores and micropores. The high steaming stability of the mesoporous silica of JLU-20-S is possibly related to synergistic advantages of both high-temperature synthesis and preformed zeolite nanoclusters in the synthesis of ordered mesoporous silica materials, and notably JLU-20-S show much more microporous volume than JLU-20 without micropores, which is a direct evidence that the use of preformed zeolite nanoclusters will introduce the microporosity in the mesoporous silica materials.