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Synthesis, Characterization And Application Of Spherical Heteroatomic Mesoporous Materials

Posted on:2011-03-22Degree:DoctorType:Dissertation
Country:ChinaCandidate:D LiangFull Text:PDF
GTID:1101360305471340Subject:Chemical Engineering and Technology
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During the past ten years, there are several hot focuses about the mesoporous materials such as functionalized hybrid materials and other new mesoporous molecular sieves, bioinspired method and other new synthesis routes, biological medicine and other new applications. Among these materials, SBA-15 is an important mesoporous material due to its regular porous structure (2-30nm) and high hydrothermal stability. It is very significant for the further development of SBA-15 to synthesize the hierarchical porous materials, prepare the modified-SBA-15 materials, and develop the new applications on separation, catalysis, etc.Generally the catalysts are manufactured with special morphology and metered diameter before practical application, because the morphology and diameter of materials can immediately affect the properties and practical applications. Membrane and sphere are two major forms in the previous reports. Spherical catalysts have been affirmed to be advantageous in some reactions owing to the highest surface area among the different shapes. Therefore, the development of spherical mesoporous materials can improve their functionality and practicability in the future. In this presentation, there are two major topics: one is spherical morphology control of mono-porous and hierarchical porous materials, and another is spherical morphology control of modified SBA-15 materials. The originalities of these works are the direct synthesis method. Mono-porous and meso-macroporous silica spheres are directly synthesized without help of special moulds and other post-treatments. Heteroatomic SBA-15 spheres or hollow spheres are also directly synthesized in"one pot"synthesis solution. The following is the detailed experimental results.(1) The key factors and formation mechanism of SBA-15 spheres were not affirmed before this paper. The experimental results suggested: temperature, stirring conditions are the key factors to the synthesis of SBA-15 spheres; co-surfactant, inorganic salts and other factors can not prevent the formation of SBA-15 microspheres, but can affect the perfectness of spherical products; the formation of spherical SBA-15 was dependent on the spherical conglomerations and the polymerizing growth between surfactant micelles and silicate anions.(2) The spherical meso-macroporous silica monoliths with diameter of 0.2-0.5 mm were prepared via a sol-gel method and phase separation mechanism, in which the tetrabutylorthosilicate (TBOS) was used as silicon source. The triblock copolymer (P123) and polyoxyethylene octylphenol ether (OP-10) were used as templates. The results showed that the inner structure had macroporous channels (0.5μm) and mesoporous SBA-15 skeleton (4-5nm). The spherical silica monoliths with meso-macroporous structure demonstrated higher absorption efficiency and capacity for Emodin molecules than the SBA-15 spheres with mono-porous structure.(3) A facile route to synthesize modified SBA-15 spheres was suggested. Typically, SBA-15 synthesis solution was prepared using block polymer globular aggregates as template and tetraethyl orthosilicate (TEOS) as silica source. The heteratoms were directly added to the solution, and then ammonia water was used to adjust pH to the special value. The solid samples were filtered, washed, dried and calcined after hydrothermal treatment. Until now, Cu, Zn, Al, Fe had separately introduced the mesoporous SBA-15 spheres with diameter about 10μm. The spherical heteroatomic SBA-15 materials and the corresponding synthesis methods had been applied to patent's protection in China. Cu/SBA-15 microspheres via the same method displayed better performance in wet oxide degradation than the catalysts via traditional direct method and impregnation method.(4) CuAlZn-SBA-15 and Zr-SBA-15 hollow spheres were synthesized via the similar direct synthesis method, which showed that the hollow spheres were in ~10 microns diameter and the shell thickness was about 100 nanometers. The metal ammine complexes during the sol-gel process became the key factor in the formation of hollow spheres and dispersion of heteroatoms. The analysis results confirmed that the heteroatoms had been well embedded inside the siliceous matrix and partial nanocrystals appear inside of the pore channels, which can be used to explain the phenomenon that the surface area and orderliness of mesoporous catalysts decrease with the zirconium content. The analysis by in situ pyridine-absorption FI-IR spectra and NH3-TPD curves proved the weak and mid-strong Lewis acidity on the catalysts surface due to the introduction of heteroatoms.(5) The experimental results confirmed that incorporation of metal atoms into the mesoporous structure would supply the siliceous matrix with the new distinct catalytic activity. When the methanol dehydration was used as probe reaction, there was about 60% methanol converted to dimethyl ether while selectivity was 100% on the CuAlZn-SBA-15 hollow spheres. And the methanol conversion was increased to 71% and selectivity of dimethyl ether was >90% on Zr-SBA-15. When the dehydration of cyclohexanone and ethanediol was used as probe reaction, the ketal yield on Zr-SBA-15 hollow spheres were from 69% up to 84%. In comparison, the ketal yield was just about 10% with Si-SBA-15. The reaction performance will increase with the metal content, but the porous properties will be compromised with the increase of metal loading, which was not conducive to the methanol conversion.
Keywords/Search Tags:spherical, heteroatom, molecular sieve, SBA-15
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