Synthesis And Application Of Mesoporous Materials With Large Pores By Using Laboratory-designed Amphiphilic Block Copolymer As Template

Ever since M41S mesoporous materials was fabricated by the scientists of Mobil in 1992 by using alkyl quaternary ammonium salt surfactants and inorganic silica precursor. Scientists around the world have synthesized thousand kinds of mesoporous materials by different structure directing agent and precursors. Ordered mesoporous materials have attracted considerable attention owing to their distinctive properties such as high surface areas, diversity mesostructures, uniform and tunable pore sizes, different kind of framework composition. Are great potential for applications in catalysis, sensors, adsorption and separation, gas storage, energy storage, energy storage and drug delivery.In recent years, a number of papers relate to the synthesis of different kind of mesoporous materials are published. Generally, mesoporous materials can be synthesized by two strategies, "soft template" and "hard template" methods. For hard template method, mesoporous materials can be obtained by the following procedures: precursor molecules are filled into the pore channels of the hard template, then mesoporous materials will obtained after removed of the hard template. For soft template method, mesoporous materials are templated by structure-direction agents. Hard template method is rather complicated, need a hard template before fabricated process. So scientists are devoted to the fabrication of ordered mesoporous materials by soft template route. The most common soft template is non-ionic surfactants, they has the advantage of low toxicity, low cost. By changing the ratio of template and precursor, a large number of mesoporous materials with various topography and framework composition have been synthesized, including mesoporous silica, carbon and metal oxide. Most of the traditional synthesis methods rely on the use of commercial Pluronic triblock copolymers like PEO-PPO-PEO (P123 and F127) as templates, the pore size (usually small than 7 nm) and pore wall thickness is small owing to the low molecular weight, which will limit the application involved large object, such as sensing and adsorption of biomacromolecules, bulky molecular catalysis. Accordingly, design of new template for the synthesis of new large-pore ordered mesoporous materials is in much demand.The atom transfer radical polymerization (ATRP) approach was used to prepare amphiphilic block copolymers with different molecular weight in this paper. By using the block copolymers as structure directing agent, phenolic resin (resol) and NbCl5 as precursors, large pore polymer-niobia composite and crystallized niobia spheres were prepared, which can be used as solid acid catalysts and sensors. Solvent evaporation induced aggregating assembly (EIAA) was employed to prepare mesoporous carbon with tunable pore size in solution phase. Hollow carbon nanotubes and carbon nanowire were also fabricated by simply regulate the quantity of water. Mesoporous silica films with large pores were synthesized via spin-coating and dip-coating approach on a series of substrates. And free-standing mesoporous silica thin film has been fabricated by simple coating-etching approachIn chapter 2, we demonstrate a evaporation induced self assembly (EISA) approach to prepare ordered large pore mesoporous polymer[phenolic formaldehyde (PF) resin]-niobia composite spheres with the space group p6mm by using a laboratory-made amphiphilic block copolymer PEO117-b-PS186 as a template, resol and NbCl5 as precursors, THF as a solvent, and adding small amount of HNO3. The mesoporous materials have large-and uniform pore size (11.0 nm). The size of the spheres is hundreds nanometers to a few micrometers. Surface area and pore volume are 136 m2/g and 0.13 cm3/g, respectively. This is the first time to fabricate spherical morphology via EISA approach. The energy dispersive X-ray spectroscopy (EDS) mapping images recorded on a single mesoporous polymer-Nb composites show that the C, O,and Nb elements are homogeneously distributed in the entire spheres. Wide-angle XRD (WA-XRD) patterns indicating an amorphous feature. The polymer(PF)-niobia composite spheres can be used as solid acid catalyst for esterification reaction. The catalytic conversions were sustained higher than 96% in the first 3 cycles of esterification reactions in presence of polymer (PF)-niobia composite spheres. After 8 cycles of catalytic reactions, the conversion still preserved at 69%In chapter 3, we changed the ratio of resol and NbCl5 precursors. And crystalline mesoporous niobia spheres were fabricated by using a laboratory-made amphiphilic block copolymer PEO117-b-PS221 with higher molecule weight as a template, resol and NbCl5 as precursors. We propose a formation mechanism of mesoporous spheres. At the first stage, the water-insoluble PEO-b-PS dissolves well in acidic THF/H2O/resol/NbCl5 solution with a very high volume ratio of THF (90%). Along with the fast evaporation of THF at 50℃, the hydrophobic PS segments of the block copolymers tend to aggregate to form rod-like micelles with PS block as a core surrounded by PEO shells. Meanwhile, since the resol precursors can interact with the PEO segments of PEO-b-PS templates by hydrogen-bonding interactions and associate with inorganic Nb5+species by chelating, the PEO domains are decorated by resol and Nb5+species, forming cylindrical core-shell micelles with rod like PS microdomains covered by the resol/Nb5+composites. Therefore, the resol molecules serve as a glue which binds Nb5+species around the PEO domains, preventing the macroscopic phase separation of PEO-b-PS and Nb5+species. During the evaporation, the cylindrical core-shell composites can gradually pack together to form mesostructures. As THF further evaporates, PEO-b-PS/resol/NbCl5 composites become more insoluble because of the strong hydrophobic nature of the PS segments, and thus the flexible core-shell composite micelles tend to bend and aggregate as a result of the interfacial force of water and THF, to form composite particles with irregular shapes. A phase separation of the composites occurs from the water-rich liquid phase, and the composite micelles can further be packed to form spherical particles on the interface of the water-rich solution to reduce curvature energy. By the subsequent heating at 100℃, the resol molecules thermally polymerize into PF polymers, which fixes the composite spheres and the niobia oligomers can be further condensed, forming a "reinforced-concrete" framework structure. By pyrolysis at 350℃ in N2, the PEO-b-PS templates are selectively decomposed, leaving behind uniform mesopores in the polymer-Nb composites with well-retained spherical morphology. During the further treatment at 550℃ in nitrogen, niobia species can be crystallized without the structure collapse as a result of the support of the PF-derived carbon. At last, ordered mesoporous niobia with a spherical morphology and large pore sizes can be obtained by calcination at 400℃ in air to remove the carbon species, giving rise to ordered mesoporous crystalline niobium oxides. The diameter of the spheres ranges from around 200 nm to 1.0μm. The mesoporous crystalline niobia shows a bimodal pores distribution. The large pore centered at approximately 11.4 nm is probably from PEO-b-PS molecules, smaller than the pore diameter of polymer (PF)-niobia composite spheres, attribute to the shrinkage of framework during heat treatment. The small one at about 3.5 nm is mainly attributed the removal to carbon species in the carbon-Nb composites after the calcination in air at 400Κ. The BET surface area and pore volume of the mesoporous Nb2O5 spheres are 131 m2/g-1 and 0.26 cm3/g-1, respectively. Wide-angle XRD and selected-area electron diffraction (SAED) indicating the formation of crystalline niobia (JCPDS No.07-0061). The NbCl5/resol weight ratio is crucial for the ordered mesoporous niobia spheres. A series of control experiments reveal that the optimized ratio is 1-3:1. Using excess resol, ordered mesoporous PF-Nb composites with similar spherical morphology and mesostructure can be obtained, but upon the removal of PF-derived carbon species by calcination in air, the mesostructures are unstable and collapsed because of insufficient cross-linking in the niobia frameworks. When the NbCl5/resol ratio is too high or without the addition of resol, the ordered mesostructure from the selfassembly process could not be formed, because the interaction between the template and Nb source is too weak without the assistance of the resol. A novel biosensor was fabricated by immobilization of Hb in the mesoporous niobia spheres. It exhibits excellent electrochemical sensing of H2O2 with a high sensitivity and fast response time as a result of the short mass diffusion length, large pores size, and high surface area.In chapter 4, Solvent evaporation induced aggregating assembly (EIAA) was employed to prepare 2D hexagonal and worm-like mesoporous carbon with tunable pore size by using laboratory made PEO117-b-PS89, PEO117-b-PS198; and PEO117-b-PS264 copolymer as structure-directing agent, resorcinol and formaldehyde as carbon precursors, and adding small amount of aqueous ammonia. The pore size range from 11.0-36.3 nm, surface area and pore volume are up to 880 m2/g and 0.59 cm3/g, respectively. When the amount of water and THF was increased in synthesis process, hollow carbon nanotubes were prepared. The diameter of the pore is 33.7 nm, surface area and pore volume are 880 m2/g and 0.59 cm3/g, respectively. With the further increasement of water and THF, carbon nanowires were obtained. The diameters of the nanowire are much smaller than hollow carbon nanotubes. A mechanism which involves a good solvent (THF) evaporation induced continuous formation and aggregating by different forms of the rod-like carbon/copolymer micelles on the interface of water-rich phase is proposed. The rod-like carbon/copolymer micelles were close packing into 2D hexagonal mesoporous carbon when the water volume is low in the system. As the water volume was increased, the concentration of micelles was decreased, to form single hollow carbon nanotubes. Carbon nanowires were obtained due to micelle to reverse-micelle transition process in the solution with very high ratio of water.In chapter 5, large pore mesoporous silica films were preapared via spin-coating and dip-coating approach on a series of substrate by using high molecule weight PEO117-b-PS198 as structure directing agent, tetraethyl orthosilicate (TEOS) as silica precursor.. The diameter of the pore is 19.3 nm, surface area and pore volume are 342 m2/g and 0.40 cm3/g, respectively. Free-standing mesoporous silica thin films were obtained by a simple coating-etching approach to detach with substrates.In Chapter 6, the whole thesis is summarized.