Synthesis And Characterization Of Novel Mesoporous Molecular Sieves Templated By Block Copolymers

In 1992, the Mobil Scientists reported the synthesis of a new family of inorganic/organic hybrid materials by the use of self-assembled surfactant molecular aggregates as the structure-directing agents. After removing the organic species, mesoporous aluminosilicates M41S were obtained with uniform pore size and high surface areas. For the past ten years, such materials have attracted considerable attention in the areas such as separation of large molecular, biosensors, catalysis, adsorption, microelectronics, optics, and fabrication of novel nano-objects because of their uniform and adjustable pore properties (pore sizes and pore structures), rich surface functional groups, and designable morphologies (film, fiber, sphere, etc.). However, the low hydrothermal stability of M41S and the expensive surfactants and organic silica precursors employed during synthesis limited their advanced uses in such areas. The successful synthesis of highly ordered, highly stable, large pore (5-30 nm) hexagonal mesoporous silica SBA-15 materials by using commercial triblock poly (ethylene oxide)- poly (propylene oxide)- poly (ethylene oxide) (PEO-PPO-PEO) block copolymers under acidic conditions opens up new possibility in materials research. For future applications of functional and economic mesoporous materials, it is important to search new type of templates, investigate the relationship between the structure of templates and the property of resulted mesostructures, and synthesize advanced mesoporous materials with new structure and properties.The current contribution concerns the synthesis of novel mesoporous materials templated by nonionic block copolymer surfactants. A series of new mesoporous materials with different structures (cubic, hexagonal, lamellar and vesicles), various components (silica, carbon), adjustable pore sizes (2-50 nm) and designable morphologies have successfully been synthesized by combining the concepts of sol-gel chemistry, colloid science and supramolecular self-assembly. The structure-property correlation, the temperature and ionic strength effect in the synthesis of mesoporous materials have been studied systematically in this study.We have studied the synthesis of mesoporous silica structures by using commercial block copolymers with PEO as hydrophilic part and poly(butylene oxide) (PBO) as hydrophobic moiety, including six triblock and one diblock copolymer surfactants. Highly ordered, hydrothermally stable, caged cubic mesoporous silica structures (Im3m) FDU-1 with unusual large pore size (12 nm) have been synthesized by using EO₃₉BO₄₇EO₃₉ as a structure-directing agent, this is the largest pore size for cubic silica structures reported up to date. Similar cubic silica structures have been obtained in the presence of EO43BO14EO43 with small pore size (2.4 nm). Hexagonal silica mesostructures have been synthesized from EO₁₇BO₁₄EO₁₇ and EO₁₇BO₁₀ with pore sizes of 3.8 nm and 6.0 nm, respectively. Under low temperature (0°C) and strong acidic condition (6M HCl), large pore (10 nm) silica vesicles have been obtained for the first time in the presence of EO15BO45EO15 with unusual large hydrophobic/hydrophilic volume ratio; a transition from vesicle to hexagonal structure has been observed by changing the Si/copolymer ratio and other reaction parameters. For block copolymers EO₁₃BO₁₁EO₁₃ and EO₃₄BO₁₁EO₃₄ with relatively large critical micelle concentrations (CMC), only disordered silica mesostructures were obtained with relatively uniform pore size of 2.4 and 2.0 nm, respectively. Several conclusions can be drawn from this study: 1, the pore size of mesoporous materials is dependent on the molecular weight of hydrophobic part of block copolymer templates and related to the type of mesostructure; 2, for block copolymer surfactants with the same hydrophobic molecular weight, the pore size of mesoporous materials is much larger for PBO than that for PEO, 3, for the same block compositions, the pore size of mesoporous materials templated by diblock copolymer is much larger than that templated by triblock copolymer surfactants. By comparing the CMC of triblock copolymers and the resulted silica mesostructure, we proposed that the CMC and critic micelle temperature (CMT) can be used as important criteria in the design of the structure and the synthesis approach of mesoporous silica precipitated from solutions.By studying the temperature and salt effect in the synthesis of mesoporous materials with block copolymers, we proposed that the use of"salting-out"inorganic salts can dramatically widen the syntheses domain (in temperature, surfactant concentration, etc.) and broaden the range of surfactants that can be utilized to produce highly ordered mesostructures. By utilizing this concept, highly ordered mesoporous silica SBA-15 materials with low microporosity have been synthesized under low temperature and low surfactant concentrations; the quality of resulted mesostructure or pore size distribution can be greatly improved in the cases of EO₄₃BO₁₄EO₄₃, EO₁₃BO₁₁EO₁₃, EO₃₄BO₁₁EO₃₄, EO₁₇BO₁₄EO₁₇, EO₉₉PO₇₀EO₉₉ and EO₁₃₂PO₅₀EO₁₃₂ block copolymers. Inorganic salts also have great influence in the morphology control of mesoporous materials templated by nonionic block copolymers. By using inorganic salts, we have synthesized SBA-15 with spherical morphology at low temperature, while highly ordered SBA-15 materials with 100% rod-like morphologies were obtained at relatively higher temperature. These straight rods are quite uniform in length (1-2μm), the mesopore channels are parallel to the long axes of the rods. By studying the particle growth of these SBA-15 rods, a phase separation mechanism of the formation of mesoporous powders in solution is proposed. The formation of mesoporous powders can be divided into three stages: 1, cooperative assembly in solution to give of inorganic-organic composites; 2, further condensation of inorganic species to give a liquid crystal like phase and further liquid-liquid phase separation; and 3, the formation of final mesostructure from separated liquid crystal like phase. The competition between the total energy of self-assemblyΔG and the surface tension F can be related to the formation of final mesostructure and morphology. Based on general concepts in sol-gel chemistry and the proposed phase separation mechanism, the effects of inorganic salts on the morphology of mesoporous materials can be interpreted at the colloidal level.Highly ordered cubic mesoporous silica SBA-16 with millimeter spherical morphology (2-4 mm in diameter) was successfully synthesized by use of inorganic salts. We have also reported the first synthesis of large pore (7.4 nm), cubic mesoporous silica single crystals with exclusively uniform rhombdodecahedron shapes ( 1μm) and ¹00% crystal yield in the presence of EO132PO50EO132 templates. These 12 faces can be indexed to {110} planes. For thin sections of each crystal face, the TEM image along only [110] direction is observed, moreover, at each crystal face, the diagonal of the rectangular repeating unit is parallel to the crystal edge, in accordance with an ideal crystal model with Im3m symmetry. The unit cell is propagated throughout the faceted particles without twinning or apparent dislocations and fault planes, unambiguously confirming that the particles are perfect single crystals.By utilizing mesoporous silica structures as the hard templates, mesoporous carbons with controlled morphologies and structures have been synthesized. We have synthesized highly ordered hexagonal mesoporous carbon materials with fiber-like, plate-like, rod-like and donut-like morphologies by using SBA-15 silica templates. N₂ sorption analysis results reveal that these mesoporous carbon materials with controlled morphologies have very large surface area (up to 1900 m²g-1) and pore volume (up to 2.23 cm³g-1), suggesting that these carbon materials are very valuable in future applications. Moreover, cubic mesoporous carbon materials have been successfully synthesized from SBA-16 and FDU-1 silica templates with cage-like structures by methods of improving the quality of mesoporous templates and enlarging the window size of caged pore structures.We have extended previous work from water-rich phases to oil-rich phases. By the combination of sol-gel chemistry and reverse emulsion chemistry, siliceous hollow spheres (1-4μm) with ultra large mesopore wall structures (～50 nm in pore diameter,～200 nm in wall thickness) have been synthesized with a high surface area (674 m2/g) and pore volume (1.25 cm³/g). In non-polar solvent systems, a reverse amphiphilic mesophases approach to create hierarchically arrayed silica nanorods has been developed. The cylinder nanorods with uniform diameter (10 nm) generated from highly ordered two-dimensional (2D) reverse hexagonal mesophases are aligned within lamellar macrostructures (～150 nm). The diameter size of silica nanorods is small and can be adjusted from 9 to 15 nm by using different surfactants containing PEO moiety and their concentrations. The calcined material has a relatively small surface area of 275 m²/g. This approach may provide a new pathway to the fabrication of nano-objects that might be applied in nanoscale devices.