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Mesoporous Oxide Synthesis And Application Of The Hydrothermal Stability Of Silicon-based Materials And New Types Of Mesoporous Carbon Materials

Posted on:2011-01-07Degree:DoctorType:Dissertation
Country:ChinaCandidate:Q LiFull Text:PDF
GTID:1111360305997258Subject:Inorganic Chemistry
Abstract/Summary:PDF Full Text Request
Since the discovery of M41S family of mesoporous silica materials, mesoporous materials have been applied in research fields, such as electrochemisty, catalysis, adsorption, biology, and so on, which deriving from their high specific surface areas, large pore size(>2nm) and different mesostructures. Generally, mesoporous materials can be synthesized by two strategies, soft template and hard template methods. For soft template method, mesoporous materials are templated by structure-direction agents, including cationic surfactant, nonionic surfactant and anionic surfactant. For hard template method, mesoporous materials can be obtained by the following procedures:precursor molecules are filled into the pore channels of the mesoporous silica (hard template), then after heat treatment at high temperature, the hard template can be removed by HF or NaOH etching. In recent years, mesoporous silica-based materials have been extensively used as catalysts and catalyst supports. However, how to improve their mesostructural stability is still a "hot point" in the activation process under 100% steam. Moreover, many people pay a great deal of attention to catalysis, energy and environmental problems, so it is very necessary to design and synthesize novel carbon-based materials for these aspects mentioned above.As a result, in this thesis, we investigate the hydrothermal stability of mesoporous celluar silica foams (MCFs) and Al-SBA-15 materials in 100% steam for improving their performance as catalysts or catalyst supports. The thermal stable and highly ordered mesoporous ZrO2/FDU-15 composites are synthesized by triconstituent co-assembly strategy, which show excellent catalytic performance on oxidative dehydrogenation of ethylbenzene. Furthermore, spherical mesoporous cellular carbon foams (C-MCF) are synthesized by using spherical mesoporous celluar silica foams as hard template. This material is a promising candidate for electrochemical capacitor.In chapter two, we investigate the hydrothermal stability of MCFs in detail. A series of MCF materials with different structural parameters are synthesized by adjusting the synthesis conditions including aging temperature, calcination temperature and the presence of salts. The mesostructures of MCFs calcined at 550℃can be well-retained in 100% steam at 600℃for 12 h. However, their mesostructures completely collapse after the steaming temperature increased to 800℃. For further improving the hydrothermal stability of MCFs, their calcination temperatures are increased to 900℃. In this condition, the MCF materials can endure the 800℃-100% steam for more than 12 h. The results indicate that increasing the calcination temperature is an effective method to improve the hydrothermal stability of MCFs. It is concluded that 3-D disordered MCFs show different structural variations during the high temperature steam treatments from 2-D ordered hexagonal SBA-15 materials. The pore size, window size and wall thickness were unaltered for the steam treated MCFs, while the pore size decreases and the pore wall thickness become thicker for SBA-15.In chapter three, we systematically investigate the hydrothermal stability of Al-SBA-15 materials in 100% steam. The experimental results show the introduction of Al species can obviously improve the hydrothermal stability of SBA-15. The Al-SBA-15 materials with different Si/Al molar ratios have good hydrothermal stability in 600℃-100% steam. However, the mesostructures of Al-SBA-15 with Si/Al molar ratios of 10-30 collapse completely. For the materials with Si/Al molar ratios of 40-50, their mesostructures can be well-retained in 800℃-100% steam for 6 h. After the calcination temperature is increased to 800℃, they exhibit excellent hydrothermal stability in the same condition for 12 h. In addition, it is found that abundant micropores, high calcination temperature and high Si/Al molar ratio are favorable for improving the hydrothermal stability of Al-SBA-15 materials. The steam treatment can cause the decrease of its acidic quantity, because that some four-coordinated Al species are changed into six-coordinated ones.In chapter four, highly ordered mesoporous nanocrystalline zirconia/FDU-15 composites were synthesized by using zirconium oxychloride octahydrate, resol and Pluronic F127 as zirconia and carbon sources and structure-directing agent, respectively, under evaporation induced triconstituent co-assembly strategy. The ZrO2/FDU-15 nanocomposites possess a highly ordered 2-D hexagonal (p6mm) mesostructures, high thermal stability (up to 900℃), narrow pore size distributions, high specific BET surface areas (up to 910 m2 g-1), tailorable zirconia content (up to 47 wt%), and small particle size (1.9-3.9 nm) of tetragonal crystalline zirconia confined in the matrices of amorphous carbon pore walls. The degree of pore blocking is very small even though the high embedded amount of zirconia in the carbon pore walls. Successful preparation is mainly because that the co-assembly between resol and F127 drives the co-assembly between zirconium polyoxo species and F127 surfactant, due to the hydrogen-bonding interaction between resol and zirconium species. The synthesized mesoporous composites exhibit high catalytic performance with 59.6% ethylbenzene conversion and 90.4% styrene selectivity at 350℃, O2/EB molar ratio of 0.5, which mainly derive from the synergetic interaction between the oxygen-containing groups and weak basic sites of zirconia nanoparticles and its high BET surface area and opening pore channels.In chapter five, spherical C-MCF materials were synthesized by a facile dual-templating strategy. The experimental results show that the mesostructure of the silica template is well replicated, and the synthesized S-C-MCF has high mesoporous specific surface area (1321 m2/g), hierarchically porous structure, large total pore volume (3.5 cm3/g), spherical morphology, and oxygen-containing groups modified surface. As an electrode material for EDLC, these structural characteristics and surface property endow this carbon material a high specific capacitance of 208 F/g at the loading current density of 0.5 A/g, excellent rate capability, low equivalent series resistance, and exhibits a good cycling stability with capacitance retention of 100% over 1000 cycles. Its excellent capacity can be attributed to the high BET specific surface areas,3-D pore channels, hierarchical pore size, and the presence of abundant oxygen-containing species. It is a very promising energy-storage material.In chapter six, Carbon nitride spheres were successfully synthesized for the first time by using spherical mesostructured cellular silica foams as hard template and ethylenediamine and carbon tetrachloride as precursor. The synthesized spherical material has uniform diameters of ca.4-5μm, hierarchically three-dimensional mesostructures, a relatively high BET surface area of 550 m2/g, total pore volume of 0.9 cm3/g and micropore volume of 0.08 cm3/g. Its pore wall is composed of pyridine and benzene ring interconnected by N atoms. The nitrogen content of this material is about 17.79 wt%. In addition, its CO2 uptake at 25 and75℃is about 2.90 and 0.97mmol/g, respectively, suggesting it has excellent CO2 adsorption performance. It can be attributed to the abundant basic sites on the pore wall surface, relatively high BET specific surface area and hierarchical mesostructure. Moreover, the experiment results show this material has good cycle performance for CO2 capture. Therefore, it is a very promising material for CO2 capture in large scale.
Keywords/Search Tags:mesoporous materials, synthesis, hydrothermal stability, co-assembly, composite, oxidative dehydrogenation, mesoporous carbon, capacitor, adsorption
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