Synthesis And High-resolution Electron Microscopy Characterization Of Mesoporous Materials

Mesoporous materials possess large surface areas, narrow pore size distributions, ordered structures and flexible compositions, showing great potential in chemical engineering, biomedical applications and environment protection, and therefore have attracted considerable attention. More and more techniques have been used to character mesoporous materials, among which high-resolution transmission electron microscope (HRTEM) is the most important and popular way to visually demonstrate mesostructure. The author of this thesis synthesized three different type of mesoporous materials and characterized them using HRTEM. The HRTEM studies of the three materials are focused on the arrangement of the mesopores, the substructures in the pore walls and the distribution of supported nanocrystals, respectively. The results indicate that HRTEM is very powerful and useful for the structural identification of mesoporous materials. Periodic mesoporous organosilicas (PMOs) show unique physical chemistry properties that are quite different from those of conventional silica-based mesoporous materials. PMOs have emerged as one of the most important fields in the study of mesoporous materials. To synthesize PMO with a new structure is significant to both academic investigation and practical applications. In the meanwhile, it is important but very challenging to identify a new mesostructure. Prior to our study, the 3-D highly ordered PMOs were all synthesized from basic media with limitation in pore size of < 5 nm. The author demonstrated here for the first time a highly ordered mesoporous organosilica (ethane-bridged) with large cage-like pores, successfully synthesized under acidic media, using 1,2-bis(trimethoxysilyl)ethane (BTME) as organosiloxane precursor and a triblock copolymer surfactant (EO106PO70EO106, Pluronic F127) as template. HRTEM images and corresponding ED patterns revealed that this material was assigned to have a face-centered cubic structure with space group of Fm-3m. Moreover, it consists of highly crystalline twin bands with the same cubic close-packed structure (ccp) and between the cubic twins, hexagonal close-packed (hcp) domains are often found as an intermediate phase. In the same system, the author investigated the effects of different siliceous on mesostructures. By adjusting the ratio between the two siliceous sources (BTME and TEOS), a series of mesoporous silica materials were prepared. It was concluded from HRTEM results that the introduction of organic ethane groups favors the formation of a relatively compact phase and when the organosilane content exceeds 25% in total silica precursor, the phase transition from Im-3m to Fm-3m will take place. Although the arrangement of the pores is ordered in mesoporous silicas, the connectivity of the atoms in the pore wall is disordered. Thus, mesoporous silicas are close to amorphous and different from crystals in term of physical chemistry properties, which severely limits their applications in catalysis. It can be expected that both the hydrothermal stability and the catalytic activity of mesoporous silica will be greatly improved by crystallizing its pore wall. However, the existing methods for pore wall crystallization called "secondary crystallization"inevitably destroy the mesostructure and the resulting materials are always mixtures of mesoporous silica and zeolite. In this thesis, an "improved secondary crystallization"method was firstly proposed to treat mesoporous silica SBA-15. The key point of this method is the filling of carbon in the pores of SBA-15 before secondary crystallization. Due to the presence of carbon in the pores, the mesostructure was well maintained during the flowing treatment and "phase separation"was avoided. The electron diffraction pattern of the obtained material showed a clear diffraction ring possibly that had never been observed in amorphous silica materials, suggesting that the connection of framework atoms are ordered in certain degree. This conclusion was also supported by the results of other characterization techniques including XRD, TEM, ED, FT-IR and NMR. The obtained material shows much better hydrothermal stability than conventional SBA-15, as evidenced by N2 adsorption experiments. Mesoporous materials can act as the host matrix for various guest materials. Confined by of the nanosized pores, the guest molecules are highly dispersed and through interacting with the host material, they may show some properties different from those of bulky materials. The synthesis and applications of various nanocrystals have emerged as one of the most important fields in material science. The properties of these nanocrystals are closely related to their particle size, shape, crystallinity and aggregation degree. To keep nanocrystals well dispersed without aggregation and to improve their chemical stability, as required in most applications, people usually load nanocrystals into an inert matrix e.g. mesoporous silica. A successful incorporation of nanocrystals in mesoporous silica matrix can well maintain the physical and chemical properties of nanocrystals and improve their long-term stability. Moreover, the obtained nanocrystals/mesoporous silica composites are still porous with large surface area and well-defined nanopores, which can be used as multi-functional materials. The dominant strategy for loading nanocrystals onto mesoporous silica is to in situ grow the nanocrystals inside the pores of the mesoporous silica, which suffers from some drawbacks, such as lownanocrystal loading and irregular particle morphology. Herein we describe a simple surface modification scheme that enables the incorporation of nanocrystals in the mesoporous silica matrix. The whole procedure consists of two steps. First, the surface of mesoporous silica is modified by particular functional groups that have suitable interactions (hydrophobic interaction or covalent bond) with the ligands on nanocrystals. Second, the modified mesoporous silica is mixed with nanocrystal solution, followed by evaporating the solvents under vacuum. Since the nanocrystals are externally synthesized, their morphologies and properties could be accurately controlled. Through this approach, we have successfully incorporated magnetic γ-Fe2O3 nanocrystals and Au nanocrystals in mesostructured cellular foam (MCF), as well as Au nanocrystals in highly ordered FUD-12. HRTEM images clearly indicated that the nanocrystals were uniformly distributed in the mesoporous matrix with a high loading and their regular particle morphology was well maintained. Moreover, the nanocrystals were mainly encapsulated in the mesopores instead of adhered on the the outer surface of mesoporous silica particles, as evidenced by the TEM characterization.